Overall Design Summary. We propose for prodromal patients to be randomized to D-serine vs placebo for 16 weeks. To insure that all subjects have the opportunity to receive D-serine, there will be an optional 16 week cross-over trial on the alternate study medication. No subject will be on D-serine for longer than 16 weeks. Admission criteria, Assessment Procedures, and Study Design will be the same across all sites. The procedures and timeline are shown in Table 1. The procedures and timeline are the same for the initial randomized 16 week trial and the optional cross-over trial on the alternate study medication. If patient's opt for the 16 week treatment on the alternate medication, we will use their assessments from end of initial treatment as baseline for 16 week treatment on alternate medication. Subjects will be seen for two preliminary visits, then once in treatment, subjects will be seen weekly for the first 5 visits then biweekly thereafter. A safety blood and urine collection will be done on day 3 (3 days after the start of study medication). Vital signs and weight, blood draw and urine collection for safety measures, urine pregnancy test and urine for toxicology will be repeated throughout treatment. Adverse effects ratings and symptom assessments will be repeated at each visit. Neuropsychological assessment and optional "Biomarker study" visual, auditory and ERPs tasks will be administered during one of the two preliminary visits then again at study endpoint. Any patients who convert to frank psychosis will be referred/offered immediate treatment.