We examined the effects of long-term clozapine treatment, concurrent treatment with beta-adrenergic antagonists, and clozapine-induced weight gain on serum glucose and lipid measures. Fifty subjects met the DSM-III-R criteria for schizophrenia or schizoaffective disorder, participated in a 10-week, double-blind comparison of haloperidol and clozapine and a 1-year, open-label clozapine trial, and had available serum glucose and lipid levels. Weight and glucose, and lipid laboratory values were measured at the baseline and throughout the double-blind and year-long study. There were significant increases in serum triglyceride, total cholesterol, and glucose levels during the course of clozapine treatment. There were no significant changes in high-density lipoprotein (HDL) or low-density lipoprotein (LDL). Propranolol and atenolol had additive effects on changes in the total cholesterol and triglycerides, with propranolol having the most pronounced effects. Propranolol and atenolol had no significant effect on the serum glucose levels. There were significant correlations between the triglyceride and HDL level changes and clozapine-associated weight gain during the study. There were no significant correlations between the change in serum total cholesterol, LDL, or glucose and weight gain. Clozapine therapy has adverse effects on glucose and lipid homeostasis, with clozapine-induced changes in serum glucose likely due to the inherent pharmacological properties of clozapine. Concurrent beta-adrenergic receptor antagonist treatment may have an additive effect on serum lipids, and clozapine-associated weight gain also plays a modest role in triglyceride increases.