We have observed seven initially obese individuals who, during the course of a strenuous weight-reduction program, developed diabetes mellitus: non-insulin-dependent diabetes mellitus in five cases and insulin-dependent diabetes mellitus in two cases. None had any sign of prior diabetic symptoms. Although weight reduction is encouraged in obesity, crash diets without proper medical surveillance may have deleterious effects. This sequence of induction of diabetes has not previously been reported in the medical literature. The metabolic situation in extremely low-calorie diets may be comparable to that in starvation. An attempt is made to explain our observation concerning the induction of a diabetic state during such diets, on the basis of increased insulin resistance in states of starvation and anorexia nervosa, with a concomitant role in stress hormones.