The aim of this study was to assess the influence of medical and socioprofessional factors on return to work after myocardial infarction. The authors studied a continuous series of 174 patients with an average age of 51.3 years, all of whom were active before their illness. The average follow-up period was 33 months. One hundred and thirty of the patients (75%) returned to work. The only clinical factors predictive of not returning to work were older age short exercise time and fall in blood pressure on exercise. On the other hand, nearly all socioprofessional factors, social class, type of occupation, size of company, length of employment in their company, physical stresses related to their occupation, were related to return to work. The average time before returning to work was 5.5 +/- 1 month. Though certain immediate criteria of severity of infarction such as previous myocardial infarction or anterior wall infarction were related to a more delayed return to work. The cardiac status evaluated by complementary investigations (left ventricular ejection fraction, exercise testing and Holter monitoring) was not related to the time before return to work. Of the socioprofessional factors, only difficulties related to the patients' work (modification or change of job) were associated with a more delayed return to work. Forty-four patients (33.8%) returned to work after a change in working hours (28 patients), the tasks involved (20 patients) or position (7 patients). Only the lower socioprofessional classes, independent workers and extremes of age could benefit from these measures.