We examined factors related to relative submaximal exercise performance during Phase IV exercise training in 23 patients (17 men, 6 women, mean age 70 +/- 13[SD] years) following completion of a standard 12 week program of Phase II cardiac rehabilitation. Average attendance at scheduled sessions was 70 +/- 15% during a mean duration of 2.9 +/- 2.4 years after Phase II completion. Median submaximal activity during Phase IV exercise training was identical to that at Phase II completion (5.7 METs), but with wide variation in performance over time: 10 patients remained within 1.0 MET of end Phase II effort performance (mean +0.1 METs), 8 increased exercise performance by more than 1.0 MET (mean +6.7 METs), and 5 decreased exercise performance by more than 1.0 MET (mean -1.9 METs). By analysis of variance, both older age (P = 0.001) and longer duration of Phase IV participation (P = 0.015) were independent predictors of decreasing effort tolerance during followup. In contrast, neither end Phase II submaximal effort capacity nor level of program attendance were significant independent predictors of Phase IV exercise progress. The effects of age and duration of program participation on decreasing effort capacity in this setting can be explained in part by additional co-morbid events over time in older patients.