This progress report on the prospective randomised study of the effect of coronary bypass surgery on prognosis presents the results of a three-year follow-up for all patients as well as the results of those followed up for three to five years. The 768 patients studied were men aged under 65 with mild to moderate angina pectoris, at least two-vessel disease and good left ventricular function. 373 patients were randomized to medical treatment and 395 patients to surgical treatment. Although 69 "medical" patients were subsequently operated on and 27 "surgical" patients were not operated on they were not excluded from the analysis, and the group randomised to coronary bypass surgery was compared with the group randomised to no surgery. The policy of surgery was associated with significantly better five year survival than that of no surgery, the rates being 93 x 5% and 84 x 1%, respectively. The subgroups of patients with left main disease (92 x 9% v. 61 x 7% five year survival) and three-vessel disease (94 x 9% v. 84 x 8%) benefited most from the policy of surgery. Symptomatic improvement, consumption of beta-adrenergic blocking agents, and exercise performance were also significantly better for the surgical group than for the medical.