Background: Vitamin D deficiency is a re-emerging epidemic, especially in minority populations. Vitamin D is crucial not only for bone health but for proper brain development and functioning. Low levels of vitamin D are associated with depression, seasonal affective disorder, and schizophrenia in adults, but little is known about vitamin D and mental health in the pediatric population. Methods: One hundred four adolescents presenting for acute mental health treatment over a 16-month period were assessed for vitamin D status and the relationship of 25-OH vitamin D levels to severity of illness, defined by presence of psychotic features. Results: Vitamin D deficiency (25-OH D levels <20 ng/ml) was present in 34%; vitamin D insufficiency (25-OH D levels 20-30 ng/ml) was present in 38%, with a remaining 28% in the normal range. Adolescents with psychotic features had lower vitamin D levels (20.4 ng/ml vs. 24.7 ng/ml; p = 0.04, 1 df). The association for vitamin D deficiency and psychotic features was substantial (OR 3.5; 95% CI 1.4-8.9; p <0.009). Race was independently associated with vitamin D deficiency and independently associated with psychosis for those who were Asian or biracial vs. white (OR = 3.8; 95% CI 1.1-13.4; p < 0.04). Race was no longer associated with psychosis when the results were adjusted for vitamin D level. Conclusions: Vitamin D deficiency and insufficiency are both highly prevalent in adolescents with severe mental illness. The preliminary associations between vitamin D deficiency and presence of psychotic features warrant further investigation as to whether vitamin D deficiency is a mediator of illness severity, result of illness severity, or both. Higher prevalence of vitamin D deficiency but no greater risk of psychosis in African Americans, if confirmed, may have special implications for health disparity and treatment outcome research.