The objective of this study was to evaluate whether the relationship between self-care behavior and metabolic control is comparable in patients with non-insulin-dependent diabetes mellitus (NIDDM) on insulin and not on insulin. We studied 84 NIDDM patients hospitalized for an elective admission in Washington University's Model Demonstration Unit. At admission, patients reported the frequency of exercise, blood glucose monitoring, and meal skipping for the previous 2 wk. Metabolic control over the previous 8-12 wk was determined from glycosylated hemoglobin assays. In cross-sectional analysis controlling for patient sociodemographic and health characteristics, glycosylated hemoglobin levels were positively related to meal skipping (P = 0.0008) and negatively related to the frequency of blood glucose monitoring (P = 0.0025). Self-care behaviors explained 26% of the variance in glycosylated hemoglobin levels in NIDDM patients. Multivariate modeling demonstrated no significant interaction effects between insulin treatment and self-care on metabolic control. In conclusion, these findings support the clinical significance of self-care activities for metabolic control in NIDDM patients, particularly meal skipping and blood glucose monitoring.