PURPOSE Vitamin D deficiency is described as a modifiable risk factor for the incidence of and mortality in many common cancers; however, data in Hodgkin lymphoma (HL) are lacking. PATIENTS AND METHODS We thus performed a study measuring pretreatment vitamin D levels in prospectively treated patients with HL and correlated this with clinical outcomes. A total of 351 patients from the German Hodgkin Study Group clinical trials (HD7, HD8, and HD9) were included. RESULTS Fifty percent of patients were vitamin D deficient (< 30 nmol/L) before planned chemotherapy. Pretreatment vitamin D deficiency was more common in relapsed/refractory patients than matched relapse-free controls (median baseline vitamin D, 21.4 nmol/L v 35.5 nmol/L; proportion with vitamin D deficiency, 68% v 41%; P < .001). Vitamin D–deficient patients had impaired progression-free survival (10-year difference, 17.6%; 95% CI, 6.9% to 28.4%; hazard ratio, 2.13; 95% CI, 1.84 to 2.48; P < .001) and overall survival (10-year