The penetration of oral 1000 mg/day ciprofloxacin into pleural fluid is investigated in 15 patients with exudative pleural effusion. After 4 days of ciprofloxacin therapy ciprofloxacin concentrations were measured in plasma and pleural exudate simultaneously by high-performance liquid chromatography (HPLC). Mean serum ciprofloxacin level was 1.58 +/- 0.91 mg/L and mean pleural exudate concentration was 1.00 +/- 0.59 mg/L. The concentrations achieved were all above the MIC90 of the majority of Gram-positive and Gram-negative pathogens. It is concluded that ciprofloxacin penetrates well into the pleural fluid.