Five adults completed this four-way randomized crossover study to compare the effects of oral treatment with ciprofloxacin, clarithromycin, and a combination of the two drugs on theophylline pharmacokinetics. The area under the concentration-time curve for theophylline during combination therapy was not different from that for ciprofloxacin alone. Beta error may explain this finding, but any real effect from combination treatment appears to be clinically unimportant.