The purpose of the study is to determine the safety and efficacy of D-serine as an early intervention treatment for the schizophrenia prodrome condition. This study is a placebo-controlled trial of D-serine in the symptomatic treatment of patients with the schizophrenia prodrome. Seventy two subjects meeting criteria for the schizophrenia prodrome will be included in this study, 24 at each site (Yale, Nathan Kline Institute and Zucker Hillside Hospital). The primary outcome measures will include symptom and neuropsychological measures. The duration of this study is two and a half years.