A single-blind, placebo-controlled study was carried out in 30 patients with chronic obstructive bronchopneumopathy to assess the effectiveness and tolerability of the theophylline derivative, bamyphylline. On entry, patients were allocated to receive twice daily oral doses of either 600 mg bamyphylline or placebo for a period of 60 days. Spirometric measurements, blood pressure and respiratory rate recordings and blood gas analyses were made at baseline and after 10, 30 and 60 days of treatment, as were assessments of clinical and objective signs, such as sibili, rales and vesicular murmurs. Plasma concentrations of bamyphylline 1 hour after the first dose of the day were monitored at the same intervals. The results showed that there was a highly significant progressive improvement in pulmonary function which was already apparent after 10 days and this was accompanied by a marked improvement in cardiac dynamics and clinical signs. Plasma concentrations of bamyphylline remained stable throughout the study period and there were no reports of side-effects or significant variations in blood chemistry.