Aims: Left heart catheterization carries a risk for cerebral complications. The aims of this prospective study were to determine the frequency and composition of catheterization-related cerebral microemboli and to detect cerebral morphological changes and acute cognitive impairment due to catheterization. Methods and results: Forty-seven unselected patients undergoing elective left heart catheterization, either by transradial or by transfemoral access, were monitored for cerebral microemboli using multi-frequency transcranial Doppler. Cerebral magnetic resonance imaging(MRI) with diffusion-weighted imaging sequences and neuropsychological assessments were carried out on the day before and the day after catheterization. A median number of 754 cerebral microemboli were detected: 92.1% were gaseous and 7.9% were solid. New cerebral lesions were observed in 15.2% of the transradial, but none of the transfemoral, catheterization patients(P=0.567). These lesions were significantly associated with a higher number of solid microemboli(P=0.016) and a longer fluoroscopy time(P=0.039). There was also a significantly higher number of solid microemboli during transradial than during transfemoral catheterization(P=0.012). Cognitive impairment following the investigations was associated with the degree of pre-catheterization cerebral MRI injury(P=0.03). Conclusion: During left heart catheterization, cerebral microemboli, especially those which are solid, may damage the brain. Cardiac catheterization may therefore pose a greater risk for the brain than previously acknowledged. Aims: Left heart catheterization carries a risk for cerebral complications. The aims of this prospective study were to determine the frequency and composition of catheterization-related cerebral microemboli and to detect cerebral morphological changes and acute cognitive impairment due to catheterization. Methods and results: Forty -seven unselected patients undergoing elective left heart catheterization, either by transradial or by transfemoral access, were monitored for cerebral microemboli using multi-frequency transcranial Doppler. Cerebral magnetic resonance imaging (MRI) with diffusion-weighted imaging sequences and neuropsychological assessments were carried out on The day before and the day after catheterization. A median number of 754 cerebral microemboli were detected: 92.1% were gaseous and 7.9% were solid. New cerebral lesions were observed in 15.2% of the transradial, but none of the transfemoral, catheterization patients ( P = 0.567). These lesions were significantly associated with a higher number of solid microemboli (P = 0.016) and a longer fluoroscopy time (P = 0.016). There was also a significantly higher number of solid microemboli during transradial than during transfemoral catheterization was associated with the degree of pre-catheterization cerebral MRI injury (P = 0.03). Conclusion: During left heart catheterization, cerebral microemboli, especially those which are solid, may damage the brain. than previously acknowledged.