The influence of long-term timolol treatment on plasma lipids was analysed in cohorts of the Norwegian timolol multicentre study. The prognostic importance of high-density lipoprotein (HDL) cholesterol concentration after myocardial infarction was also examined. One year timolol treatment was related to a significant reduction in HDL cholesterol levels, from 1.32 mmol l-1 to 1.26 mmol l-1 (P less than 0.05). After one year the HDL cholesterol levels were significantly lower in the timolol treated patients (1.26 mmol l-1) than in the placebo treated patients (1.32 mmol l-1, P less than 0.01). However, the HDL cholesterol values after myocardial infarction had no prognostic importance, and in the placebo group total mortality was the same in patients with low HDL cholesterol (less than 1.25 mmol l-1) and high HDL cholesterol (greater than or equal to 1.25 mmol l-1), respectively 15.0% and 14.8%. Timolol treatment was related to a reduction in mortality both in patients with low (24%, NS) and with high (43%, P less than 0.05) HDL cholesterol levels. Thus, any deleterious effects of timolol on serum lipids did not attenuate its protective effect on the damaged myocardium.