Vitamin D insufficiency is common in the United States, with low levels linked in some studies to higher cancer incidence, including non-Hodgkin's lymphoma (NHL). Recent data also suggest that vitamin D insufficiency is related to inferior prognosis in some cancers, although there are no data for NHL. We tested the hypothesis that circulating 25-hydroxyvitamin D 25(OH)D levels are predictive of event-free survival (EFS) and overall survival (OS) in a prospective cohort of 983 newly diagnosed patients with NHL. 25(OH)D and 1,25-dihydroxyvitamin D 1,25(OH)(2)D levels were measured by liquid chromatography-tandem mass spectrometry. Mean age at diagnosis was 62 years (range, 19 to 94 years); 44% of patients had insufficient 25(OH)D levels (< 25 ng/mL) within 120 days of diagnosis. Median follow-up was 34.8 months; 404 events and 193 deaths (168 from lymphoma) occurred. After adjusting for known prognostic factors and treatment, 25(OH)D insufficient patients with diffuse large B-cell lymphoma (DLBCL) had inferior EFS (hazard ratio HR, 1.41; 95% CI, 0.98 to 2.04) and OS (HR, 1.99; 95% CI, 1.27 to 3.13); 25(OH)D insufficient patients with T-cell lymphoma also had inferior EFS (HR, 1.94; 95% CI, 1.04 to 3.61) and OS (HR, 2.38; 95% CI, 1.04 to 5.41). There were no associations with EFS for the other NHL subtypes. Among patients with DLBCL and T-cell lymphoma, higher 1,25(OH)(2)D levels were associated with better EFS and OS, suggesting that any putative tumor 1-α-hydroxylase activity did not explain the 25(OH)D associations. 25(OH)D insufficiency was associated with inferior EFS and OS in DLBCL and T-cell lymphoma. Whether normalizing vitamin D levels in these patients improves outcomes will require testing in future trials.