Forty patients with Crohn's disease were divided into undernourished (18) and well nourished (22) groups depending on whether their midarm circumference was below or above 90% of the ideal standard. Plasma 25-(OH)D3 and the dihydroxylated metabolites, 24,25-(OH)2D3 and 1,25-(OH)2D3 were measured in the summer. Results were related to clinical and biochemical parameters and also compared with results from patients with ulcerative colitis and healthy subjects who served as controls. Plasma 25-(OH)D3 was reduced in the undernourished Crohn's group compared with the well nourished Crohn's group, who did not differ from the controls. Over 50% of the undernourished Crohn's group had evidence of secondary hyperparathyroidism and raised alkaline phosphatase concentrations, although concentrations of 1,25-(OH)2D3 were normal. The low 25-(OH)D3 concentrations related to disease activity. It is suggested that undernourished Crohn's patients who have high levels of disease activity are at risk of vitamin D deficiency, and attempts should be made to improve their vitamin D nutrition.