The effect of a multiple-dose regimen of oral ciprofloxacin (750 mg every 12 h for 11 doses) on the clearance and steady-state concentrations of theophylline in trough (predose) serum was evaluated in nine healthy male subjects, each serving as his own control. Theophylline was taken as a sustained release tablet per os in a dose of 200 mg every 12 h for 19 doses. Theophylline concentrations in serum were measured immediately before each theophylline dose. Ciprofloxacin was administered on study day 4 through the first dose of study day 8. Theophylline concentrations in serum were also measured on study days 3, 6, 8, and 10 at the following times after the first dose of each day: 0, 0.25, 0.50, 1, 2, 4, 6, 8, 10, and 12 h. Steady-state theophylline concentrations in trough serum were significantly higher during ciprofloxacin treatment (day 8) than before (day 3) or after (day 10) ciprofloxacin administration (P less than 0.01). Likewise, theophylline clearance was significantly slower (P less than 0.01) during ciprofloxacin treatment (day 8) than before it (day 3) or after it (day 10). The magnitude of ciprofloxacin-induced changes was approximately 30%. These results suggest that a multidose regimen of ciprofloxacin significantly slows the clearance of theophylline and elevates theophylline concentrations in serum.