The merit of using anaerobic threshold (AT) for prescribed exercise was evaluated in 55 out-patients with old myocardial infarction (OMI) one to 3 months after the attacks. Comparisons of optimum exercise intensities using 2 methods; either heart rate at AT (ATHR) or heart rate by Karvonen's method (KHR), were conducted. Treadmill stress test was performed in all subjects. ATHR was determined by ramp protocol and KHR by Bruce's protocol. Items measured were AT, peak oxygen consumption (peak VO2), ATHR and KHR. The results were as follows: 1. Mean peak VO2 and AT in patients with OMI were 25.8 +/- 4.3 ml/min/kg and 16.5 +/- 2.1 ml/min/kg, respectively. Mean heart rates were 117.2 +/- 11.0 beats/min at AT and 121.9 +/- 13.0 beats/min by Karvonen's method, respectively. 2. A positive correlation (r = 0.85) was noted between AT and the peak VO2. 3. A positive correlation (r = 0.61) was also noted between the peak VO2 and ATHR. 4. Correlations of KHR with peak VO2 and with ATHR were insignificant. 5. There were 6 patients with ATHR > or = KHR and 15 with ATHR < KHR. There was no significant difference between their clinical backgrounds of 2 groups. It has been thought that with KHR, about 70% of the patients with OMI are overloaded due to excessive exercise intensity. We concluded that exercise training using AT is better and more useful in view of training efficiency and safety.