Six adolescents, 12 to 15 years old, with insulin-dependent diabetes mellitus were discovered to be secretively taking extra insulin, not with the intent of improving metabolic control. Large discrepancies between reported and observed insulin requirements were noted. Psychosocial problems antedated the discovery of surreptitious insulin administration in all. Psychological testing and psychiatric evaluation revealed a variety of psychiatric conditions; depression was common. In two patients surreptitious insulin administration was believed to represent suicidal behavior. In others, it appeared to represent symptom substitution when use of other health-threatening behaviors such as recurrent ketoacidosis was made increasingly difficult through appropriate intervention. Surreptitious insulin administration may be one symptom of serious underlying psychiatric dysfunction in adolescents with insulin-dependent diabetes.