An 82-year-old man with a history of myasthenia gravis and heart failure was admitted to the hospital with respiratory failure. Aminophylline and eventually theophylline therapy were initiated to improve respiratory status. During the hospital stay, the patient developed a resistant pseudomonal pneumonia. After failure with conventional antibiotics, ciprofloxacin was initiated because of favorable sensitivity and the planned avoidance of aminoglycoside therapy. Seventy-two hours after initiation of ciprofloxacin, the patient's theophylline level rose from a steady-state baseline of 9.8 micrograms/ml to 34.7 micrograms/ml. After the theophylline dose was reduced by approximately 67 percent, the patient's theophylline serum concentration returned to baseline (10 micrograms/ml). Until more data concerning the interaction of theophylline and ciprofloxacin are available, we recommend close monitoring of theophylline serum concentrations in patients receiving concomitant ciprofloxacin.