In a double blind cross-over comparison of a single dose 450-mg slow-release aminophylline (SRA) with placebo in eleven patients with stable, reversible airways obstruction SRA produced significantly greater increase in FEV1 and vital capacity from 2 to 9 hr after administration. A second double-blind cross-over comparison of 450-mg SRA with 400 mg of choline theophyllinate (CT) in eleven similar patients showed that SRA produced a slower rise in FEV1 and VC than CT. However, the increase in spirometric readings following SRA was sustained at 9 hr after administration where the spirometric recordings after CT were falling. Whereas CT produced a peak value in plasma theophylline at 1 hr, the highest values after SRA occurred at 4 or 8 hr after administration. The plasma theophylline level at 8 hr after SRA was significantly higher than that following CT. In this single-dose study, plasma theophylline levels obtained were within the normal therapeutic range and this was not exceeded and no side effects were recorded.