Fifteen adult chronic asthmatic patients were studied on 6 consecutive days of the second week of treatment with a new sustained release theophylline formulation, and 8 were again studied after three months on the same dosing regimen (375 mg b.i.d.). Serum theophylline concentrations were maintained in the therapeutic range (peak - 19.7 +/- 5.0 micrograms/ml; trough - 13.0 +/- 3.2 micrograms/ml) throughout the 12 hour dosing interval, and were greater than 75% of the peak concentration over 8.6 +/- 2.9 h. A degree of drug accumulation was evident in that the 1-h and 5-h levels rose from 12.2 +/- 4.1 and 4.5 +/- 4.8 micrograms/ml during the second week to 16.9 +/- 4.6 and 18.4 +/- 4.5 micrograms/ml, respectively, at three months. Between-patient differences accounted for 61%-71% of the total variation in steady state theophylline concentrations. After accounting for differences due to sampling time and assay error, unexplained random, within-individual variability amounted to 11%-18% of the total. Quantitative estimation of these components of variability may be incorporated into dosage forecasting methods based on single determinations of serum concentration.