Third-generation cephalosporins in general have few adverse effects and cefotaxime (Claforan) particularly is considered to be a good choice because of the favourable side effect profile. In patients with severe renal failure there have been reports of confusion and psychosis. The manufacturer therefore recommends that half the ordinary dose should be given to patients with severe renal failure. Half the ordinary dose can still be too much. We describe a patient in hemodialysis who reacted with a reversible encephalopathy with psychosis in spite of reduced doses of cefotaxime. The plasma concentration of cefotaxime was high and the reaction was diagnosed as a dose dependent side effect.