Patient compliance with a standardized incremental regimen of inhaled anti-asthma therapy has been assessed in a large, prospective study in general practice. Urine salbutamol estimations were made in 30 patients who had the largest improvement with therapy (mean increase in FEV1 0.45 l above baseline: Responsive) and in 30 patients whose airflow obstruction failed to improve (FEV1-0.14 l: Nonresponsive). The urine salbutamol concentrations rose over the 9 month period in the responsive patients as expected with the incremental doses prescribed, and were significantly higher than urine levels in nonresponsive patients at two dose levels. Poor compliance with prescribed inhaled therapy is an important cause of persistent morbidity from asthma.