In brief: The acute effect of exercise on blood glucose was observed on 130 occasions in eight adolescents with insulin-dependent diabetes mellitus. All subjects engaged in 30 minutes of either structured aerobic exercise or unstructured recreational activity. The magnitude of decline in the blood glucose level with exercise corresponded to the preexercise blood glucose value: Higher preexercise values were associated with larger declines, while nonelevated preexercise values were associated with smaller declines. No significant differences in glycemic effect were observed between the two categories of exercise. The authors conclude that in adolescents with moderately well-controlled diabetes, recreational activities can be as effective in lowering elevated blood glucose levels as structured exercise.