In 655 individuals receiving HIV postexposure prophylaxis (PEP), drug-induced aminotransferase alterations were frequent and severe in the nevirapine-including regimen, rare and mild-to-moderate in other combinations, and always reversible. Grade 3-4 incidence in protease inhibitor or nevirapine PEP was 0.5 and 25.0 per 100 person-months, respectively. Apart from nevirapine, continuing PEP appears to be safe even in the case of aminotransferase alterations. The usefulness of routine monitoring of liver function during PEP could be re-considered.