The pharmacokinetics of ciprofloxacin were evaluated in 13 elderly patients with serious infections who were receiving 750 mg orally every 12 hours. The acute evaluations were performed within 24 hours of admission (n = 13), whereas the convalescent evaluations were performed at the end of therapy (n = 7). Serum and urine concentrations of ciprofloxacin were measured using high-performance liquid chromatography. Peak serum concentration (Cmax), terminal elimination half-life (t1/2 beta), apparent total body clearance (CL/f), and apparent volume of distribution (Vd/f) of ciprofloxacin were 5.97 +/- 2.95 mg/liter, 5.31 +/- 2.00 hours, 8.12 +/- 3.83 ml/kg/minute, and 3.63 +/- 1.91 liters/kg during the period of acute illness. Cmax and Vd/f values were moderately increased during the convalescent phase (8.56 +/- 3.43 mg/liter versus 5.87 +/- 2.25 mg/liter, p = 0.138, and 5.95 +/- 3.23 liters/kg versus 3.46 +/- 1.40 liters/kg, 0.05 less than p less than 0.1). The CL/f and t1/2 beta (four to 12 hours) values, however, were not significantly altered. The observed pharmacokinetic characteristics, which are consistent with those derived from single-dose studies in healthy elderly subjects, are markedly different from previous observations in young adult volunteers. However, acute illness does not alter the pharmacokinetics of ciprofloxacin in the elderly. Dosage alterations because of the presence of acute illness in the elderly do not appear to be warranted.