This pilot study is an eight-week, randomized, double-blind, placebo-controlled, escalating dose trial of the novel vigilance-promoting drug, modafinil, added to a stable dose of clozapine in 40 patients with schizophrenia or schizoaffective disorder. Modafinil will be initiated at 100 mg/d. After 2 weeks, the study drug may be increased to 200 mg/d and after 4 weeks, study drug may be increased to a maximum of 300 mg/d. Dose escalation will be based upon persistence of sedation versus emergence of side effects.