We performed a community-wide study in metropolitan Baltimore to examine the prognostic role of length of time between hospital admission and ventricular fibrillation or cardiac arrest (VFib/CA) complicating acute myocardial infarction (MI). Risk of developing VFib/CA was particularly marked in the first few hours after admission to the hospital. We compared 128 patients experiencing VFib/CA within 48 hours of admission and 80 patients developing these complications after more than 48 hours. Patients with "early" VFib/CA exhibited a lower in-hospital case-fatality rate than those with "late" VBib/CA (67% versus 88%; p less than .01). Likewise, of patients discharged alive from the hospital and followed for as long as six years a greater proportion of the 38 with histories of "early" VFib/CA survived than of the 11 who had experienced "late" VFib/CA during hospital admission (.05 less than p less than .10). These results suggest that whereas "early" VFib/CA may reflect transitory myocardial electrical instability, "late" VFib/CA may indicate chronic instability and thus imply a poor prognosis.