Pyrexia of unknown origin (PUO), the commonest cause of which is occult infection, represents a difficult diagnostic problem. Radiolabelled white cell scanning provides a non-invasive and potentially useful diagnostic tool in this condition, but its efficacy remains controversial. In a retrospective study, we analysed 30 111In white cell scans, performed between 1983 and 1988 in 25 patients with PUO, strictly defined as a fever of at least 3 weeks duration reaching 38.3 degrees C on more than three occasions and investigated in hospital for a minimum of 7 days. Eleven studies were on post-operative patients who, although developing fever within 1 week of surgery, satisfied the above criteria. Overall, the diagnostic sensitivity and specificity were 55% and 74% respectively, with an overall accuracy of 67%. This improved to 91% in the post-operative patients compared to an accuracy of 52% in spontaneous PUO. Accuracy did not correlate with duration of symptoms, leucocytosis or index of clinical suspicion. In conclusion, although 111In white cell scanning is not particularly useful for the investigation of spontaneous PUO, it does have a role in post-operative PUO.