Quantitative analysis of P53 protein expression was performed on paraffin-embedded tissues from 55 smooth muscle tumors of the gastrointestinal tract, using immunofluo-rescence and flow cytometry. No positive expression was found in normal smooth muscle tissues of the gastrointestinal tract. Over-expression of P53 gene was found in a significantly higher proportion in leiomyosarcomas (90%) and potentially malignant smooth muscle tumors (75%) as compared to leiomyomas (14%) (P< 0.005). The quantitation of P53 expression was found to be progressively enhanced in the sequence from leiomyoma through potentially malignant smooth muscle tumor to leiomyosarcoma (P< 0.005). It was markedly over-expressed when the mitotic counts ranged from one to more than one per 10 high power fields (P< 0.005) or the mild cytologic atypia was found (P< 0.005). The five-year survival rate was significantly higher in patients with low-expression of P53 than in those with over-expression of P53 (P< 0.005). It was suggested that P Quantitative analysis of P53 protein expression was performed on paraffin-embedded tissues from 55 smooth muscle tumors of the gastrointestinal tract, using immunofluo-rescence and flow cytometry. No positive expression was found in normal smooth muscle tissues of the gastrointestinal tract. Over-expression of P53 gene was found in a significantly higher proportion in leiomyosarcomas (90%) andable malignant smooth muscle tumors (75%) as compared to leiomyomas (14%) (P<0.005). The quantitation of P53 expression was found to be progressively enhanced In the sequence from leiomyoma through potentially malignant smooth muscle tumor to leiomyosarcoma (P< 0.005). It was markedly over-expressed when the mitotic counts ranged from one to more than one per 10 high power fields (P< 0.005) or the mild cytologic Atypia was found (P< 0.005). The five-year survival rate was significantly higher in patients with low-expression of P53 than in those with over-expression of P53 (P< 0.005). It w As suggested that P