A ten week, blinded trial of vitamin D vs. placebo in 80 patients with schizophrenia or schizoaffective disorder who have low blood levels of vitamin D and elevated blood levels of the amino acid proline. The aims of the study are to evaluate an anticipated clinical response to vitamin D supplementation including negative symptoms and cognitive deficits, evaluate safety of vitamin D supplementation for schizophrenia patients and evaluate the relationship of changes in plasma proline levels and efficacy outcomes.