A number of trials show that long stay in hospital after an acute myocardial infarction (AMI) is not necessary for many patients and that stays of three-ten days may be adequate. All patients aged under 70 years with a diagnosis of AMI admitted to the seven public hospitals in the Lower Hunter Region of New South Wales are monitored as part of the WHO MONICA Study. Between August 1984 and December 1985 of 438 hospitalised patients with a 'definite' AMI according to MONICA criteria and a clinical discharge diagnosis of AMI, 386 (88%) patients were discharged alive from hospital. Four patients had lengths of stay between 46 and 77 days and have been omitted from further analysis. The mean length of hospital stay was 13.6 days (95% confidence intervals 12.9 to 14.3 days); 74% of all patients stayed in hospital for more than ten days. The mean length of stay in the Coronary Care Unit (CCU) was 4.5 days (95% confidence intervals 4.2 to 4.8 days) with 60% staying longer than three days. Mean hospital stay varied from 10.5 to 17.4 days among the seven hospitals, although most of this variation was accounted for by three hospitals with few patients. Restricting analysis to the four hospitals with 90% of all the patients, multiple regression analysis showed that the CK enzyme levels, the evolution of Q waves on ECG, the presence of an anterior AMI and the use of nitrates and digoxin during hospitalisation were all associated with increased length of stay in hospital.(ABSTRACT TRUNCATED AT 250 WORDS)