SummaryThe predictive ability of the “condition correction factor” method of estimating total body theophylline clearance as proposed by Powell et al. was evaluated in 22 acutely ill hospitalized patients. Actual theophylline clearance was calculated while the patient was on a constant infusion of aminophylline with serum theophylline concentrations obtained at steady-state conditions. Independently, theophylline clearance was estimated for each patient using condition correction factors. The relationship between actual and estimated total body theophylline clearance was evaluated using multiple regression analysis and correlation testing. The data failed to demonstrate any significant positive correlation between actual and estimated theophylline clearance data. Because of this poor correlation, the use of one of the clearance estimation methods incorporating two serum theophylline concentrations obtained early in therapy is preferable, although less convenient. All estimates of clearance must be considered only as guides for individualization of therapy. Serum concentration monitoring and clinical response of the patient must be considered prior to adjustment of dosage.