To determine the usefulness of [18F]fluorodeoxyglucose (FDG) whole body FDG PET in the diagnosis of tumours in patients with paraneoplastic neurological syn dromes (PNS) , we prospectively studied 20 patients with paraneoplastic antibod ies in whom conventional imaging gave negative or inconclusive results for the p resence of tumour. All 20 patients had neurological manifestations compatible wi th PNS and well characterized paraneoplastic antibodies (12 anti Hu, one anti Hu and anti CV2, one anti CV2, four anti Yo, one anti Ri and one anti amp hiphysin). The mean delay between the onset of neurological symptoms and FDG PE T was 10 months (range 1-54). In these 20 patients, abnormal uptake was demonst rated in 18 patients, with some patients having abnormal signal in several areas . We observed abnormal uptake in the mediastinum (13 cases), lung (two cases), b reast (two cases), parotid gland (one case), or the cervical, supra clavicular or axillary lymph nodes (seven cases). Following FDG PET, the histological diag nosis of the tumour was made in 14 patients (small cell lung carcinoma in eight cases, breast adenocarcinoma in two, lung adenocarcinoma in two, axillary metast asis of ovary carcinoma in one, and malignant thymoma in one). Two other patient s with abnormal FDG uptake showed radiological evidence of lung cancer, but a hi stological diagnosis could not be obtained. In two other patients, initial FDG PET showed abnormal FDG uptake that was not confirmed a few months later by repe at FDG PET. In the two patients with negative FDG PET, peritoneal carcinomatos is was diagnosed in one and no tumour was found in the other. In our series, the sensitivity of FDG PET for tumour detection was >83%demonstrating a clear rol e of this technique in the management of patients with PNS. However, in our seri es, the specificity of FDG uptake was only 25%due to unexplained abnormal FDG u ptake in three patients and in abnormal FDG uptake due to a benign tumour in one patient. Over the study period, we saw 73 other patients with PNS and paraneopl astic antibodies. A tumour was demonstrated in 71 out of 73 by conventional tech niques. Since false positive and false negative results are possible with FDG PET and in most patients with PNS, the tumour is demonstrated by conventional techniques, we believe that FDG PET should be reserved, at the moment, for pati ents with well defined PNS antibodies when conventional imaging fails to identi fy a tumour or when lesions are difficult to biopsy. To determine the usefulness of [18F] fluorodeoxyglucose (FDG) whole body FDG PET in the diagnosis of tumours in patients with paraneoplastic neurological syn dromes (PNS), we prospectively studied 20 patients with paraneoplastic antibodies in in conventional imaging gave negative or inconclusive results for the p resence of tumor. All 20 patients had neurological manifestations compatible wi th PNS and well characterized paraneoplastic antibodies (12 anti Hu, one anti Hu and anti CV2, one anti CV2, four anti Yo, one anti Ri and one anti amp hiphysin The mean delay between the onset of neurological symptoms and FDG PE T was 10 months (range 1-54). In these 20 patients, abnormal uptake was demonstrated rated 18 in patients, with some patients having abnormal signal in several areas. We observed abnormal uptake in the mediastinum (13 cases), lung (two cases), b reast (two cases), parotid gland (one case), or the cervical, supra clavicular or axillary lymph nodes (seven cases). Following FDG PET, the histological diag nosis of the tumor was made in 14 patients (small cell lung carcinoma in eight cases, breast adenocarcinoma in two, lung adenocarcinoma in two, axillary metastasis as ovary carcinoma in one, and malignant thymoma in one). Two other patients with abnormal FDG uptake showed radiological evidence of lung cancer, but a hi stological diagnosis could not be obtained. In two other patients, initial FDG PET showed abnormal FDG uptake that was not recognized a few months later by repe at FDG PET In the two patients with negative FDG PET, peritoneal carcinomatos is was diagnosed in one and no tumor was found in the other. In our series, the sensitivity of FDG PET for tumor detection was> 83% demonstrating a clear rol e of this technique However, in our management of patients with PNS. the specificity of FDG uptake was only 25% due to unexplained abnormal FDG u ptake in three patients and in abnormal FDG uptake due to a benign tumor in one patieOver the study period, we saw 73 other patients with PNS and paraneopl astic antibodies. A tumour was demonstrated in 71 out of 73 by conventional tech niques. Since false positive and false negative results are possible with FDG PET and in most patients with PNS, the tumour is demonstrated by conventional techniques, we believe that FDG PET should be reserved at at moment, for pati ents with well defined PNS antibodies when conventional imaging fails to identi fy a tumor or when lesions are difficult to biopsy.