In cross-sectional studies of asymptomatic diabetic patients, multiple abnormalities in left ventricular (LV) function have been found. Long-term significance of these abnormalities is unknown because follow-up studies have not been previously performed. LV ejection fraction (EF) by radionuclide angiocardiography was examined in middle-aged control subjects (n = 44), in patients with insulin-dependent (IDDM) (n = 32) and non-insulin-dependent (NIDDM) (n = 32) diabetes mellitus at baseline and after 4-year follow-up. At baseline, all study subjects were free from cardiovascular disease. LVEF at rest did not differ between the groups at baseline. The decrease in LVEF at rest during follow-up was 1.1 +/- 1.1% (mean +/- SEM) in control subjects, 3.1 +/- 1.3% (p = NS, compared with control subjects) in patients with IDDM, and 7.2 +/- 1.4% (p < 0.01) in patients with NIDDM. At follow-up examination, abnormally low LVEF at rest (< 50%) was found in 7% of control subjects, 13% of patients with IDDM (p = NS), and in 31% of patients with NIDDM (p < 0.05). Compared with control subjects, the prevalence of an abnormal LVEF response to exercise (an increase by < 5%, or a decrease) was higher in diabetic groups at both examinations. This prevalence increased in control subjects from 10% at baseline to 26% at follow-up examination.(ABSTRACT TRUNCATED AT 250 WORDS)