The Adaptation Nursing Model provided the theoretical framework for the comparative analysis of psychological and physiologic adaptation of 211 adults representing three diagnostic groups (rheumatoid arthritis, hypertension, and multiple sclerosis). Data were collected through interviews and completion of the Mental Health Index, Health-Related Hardiness Scale, and Margin in Life. Psychological adaptation was found to be independent of diagnosis. Four predictor variables (health promotion activities, psychological distress, physiologic adaptation, and dependence on medications) significantly discriminated among the three groups and correctly classified 73.08% of the total sample. Presence of the hardiness characteristic was significantly related to psychological and physiologic adaptation, involvement in health promotion activities, and participation in patient education programs. It can be concluded that a diagnosis-specific view of psychological status is not tenable or clinically meaningful.