In a randomized, double-blind study, in which recombinant tissue plasminogen activator (rt-PA) administered at an early stage was compared with placebo in patients with suspected acute myocardial infarction (AMI), the effects on pain were studied in 312 patients. Inclusion criteria were as follows: (a) chest pain of duration less than 2 h and 45 min; and (b) age less than 75 years. Chest pain was estimated subjectively by the patients, using a 10-point numerical rating scale, at hourly intervals for the first 24 h, and by the requirement for narcotic analgesics. Compared with placebo, rt-PA treatment resulted in a 43% reduction in mean total pain score (P less than 0.0001), a 26% reduction in pain duration (P less than 0.01), and a 33% reduction in morphine requirement (P = 0.01). Fifty-seven per cent of all patients developed a confirmed AMI. In these subjects rt-PA reduced the pain score by 46% (P less than 0.001). Among patients without confirmed AMI, a 37% reduction in pain score was observed (P = 0.05). The effect on pain was most marked in patients with ST-elevation on the initial ECG. We conclude that early treatment with rt-PA in suspected AMI reduces chest pain considerably. The effect is most marked in patients with ST-elevation on the initial ECG.