32 patients with at least 3 weeks' unexplained fever and no established diagnosis after at least one week's in-hospital evaluation underwent scintigraphy after injection of 111In-labelled granulocytes. Focal infectious processes were correctly identified in 5 patients (1 dental and 4 abdominal infections). In a patient with non-Hodgkin's lymphoma the lymphomas took up 111In-granulocytes. Intestinal activity was observed in a patient eventually diagnosed as Whipple's disease. Apart from these findings, weak and slowly appearing focal tracer accumulations of uncertain significance were seen in 4 cases. So far, no sources of infection have been identified in any of the patients outside the infectious disease group with a negative scintigram during a median follow-up period of 8 months. Our results support the suggestion that the 111In-granulocyte scintigraphy method is a sensitive method for the detection of occult infections, and it may prove useful in the evaluation of patients with protracted fever of undetermined origin.