Analyses of 90 admissions to the Cincinnati Children's Hospital Center, Cincinnati, Ohio, were used to assess the effects of the routine availability of serum theophylline determinations (STD) in treating acute in-hospital asthma over three one-year review periods coinciding with the relative availability of STD. Significant differences were found between the first and third periods for the mean outpatient theophylline dose used on admission, the mean initial in-hospital intravenous dose, the mean initial oral dose during admission and the mean oral discharge dose. No statistical differences were noted in the length of hospital stay, duration of intravenous therapy or report of theophylline toxicity (despite the use of larger theophylline doses). Utilization of STD must be guided by cost-effective pharmacokinetic principles.