Whether the simultaneous administration of ciprofloxacin or tazobactam/piperacillin increases the risk of thrombocytosis is unknown. Broncho-pulmonary infection in a 50-year-old male with acute, hypertensive, intracerebral bleeding, necessitated therapy with cefpirome (2 g/day, 6 days), ciprofloxacin (800 mg/d, 11 days) and tazobactam/piperacillin (9 g/day, 11 days). Starting with the 8th hospital day, the thrombocyte count steadily increased from 410000/microl to a maximum of 1132000/microl on hospital day 16. Afterwards the thrombocyte count continuously decreased to normal values. Primary thrombocytosis and secondary causes were excluded. Since the thrombocyte count started to increase immediately after initiation and dropped immediately after discontinuation of ciprofloxacin and tazobactam/piperacillin and all other drugs were discontinued already before or were started after the nadir of the thrombocyte count, these two antibiotics were regarded causative. It is concluded that simultaneous administration of ciprofloxacin and tazobactam/piperacillin may cause marked thrombocytosis. Discontinuation of these two antibiotics results in an immediate decline of the thrombocyte count to normal values within three weeks.