Non-insulin-dependent diabetes mellitus (NIDDM) is a serious health problem among the Zuni Indians of New Mexico. In July 1983, Indian Health Service personnel initiated a community-based exercise program designed to help control NIDDM in the community. To retrospectively evaluate the effects of the exercise program, the medical records of 30 participants with NIDDM were compared with the medical records of 56 nonparticipants with NIDDM matched by age, sex, health-care provider, and duration of NIDDM. From 1 July 1983 through 1 October 1985, participants had a mean weight loss of 4 kg, whereas nonparticipants had a mean weight loss of 0.9 kg (P < .05). Participants' fasting blood glucose values dropped by a mean of 43 mg/dl, compared to a mean drop of 2 mg/dl among the nonparticipants (P < .05). Participants were significantly more likely than nonparticipants to have stopped their hypoglycemic medication (relative risk 4.2) and to have decreased their medication dosage (relative risk 2.2). These results suggest that participation in a community-based exercise program can produce significant weight loss and improvement in glycemic control among a group of Native Americans with NIDDM.