The test-retest reliability of the Minnesota Leisure Time Physical Activity Questionnaire was assessed by two administrations of the questionnaire at a five-week interval in 140 adults from a general population sample and at a two-week interval in 150 male participants in the Multiple Risk Factor Intervention Trial (MRFIT). In both study populations, there was a slight, nonsignificant drop in reported leisure time energy expenditure between the test and the retest. The largest test-retest differences in estimated energy expenditure were found among those reporting greater levels of energy expenditure. Nevertheless, in both study populations Spearman rank correlation coefficients between the test and retest were high--0.79 to 0.88 for total activity and ranging 0.69-0.86 among the light, moderate, and heavy intensity subcategories. The high reliability of the Minnesota Leisure Time Physical Activity Questionnaire corroborates its utility in epidemiologic research.