A multicentre study of cases of acute myocardial infarction was undertaken at 27 departments at teaching and non-teaching hospitals throughout Austria over a period of three years. Altogether 3397 patients were investigated. On classification of the patients according to the number of shock indicators, two comparable groups (B and C) of "mild" infarction with 0, 1 or 2 signs of shock were obtained. These low-risk groups comprised 728 patients. The mortality in group C ("mild" infarction, no streptokinase) was 17.3%, significantly higher (p less than 0.01) than the corresponding figure of 10.5% in group B ("mild" infarct, streptokinase therapy). The decrease in mortality by streptokinase therapy applied both to monitored patients as well as to those who were not monitored. Haemorrhage was a very rare complication, but somewhat more frequent in the streptokinase-treated cases, as expected. The incidence of complications such as stereocardia, asystole, and cardiac insufficiency, as well as the conversion of "mild" cases into a "severe" symptomatology was markedly reduced in the streptokinase-treated group.