The development of pellagra in a patient treated with 5-fluorouracil for malignant disease is reported. The aetiology of pellagra in this patient is discussed, and the reasons for possible under-diagnosis of pellagra in association with malignant disease considered. We report a patient who presented with the typical skin changes of pellagra. The rash, and an associated acute deterioration in cerebral function, were exacerbated by treatment with 5-fluorouracil. The importance of considering nicotinic-acid deficiency in patients with malignant disease has not been emphasized in the literature.