Fever of unknown origin (FUO) has been defined as an elevation in temperature (38°C) for at least 2-3 weeks despite intensive investigation. The value of immunoscintigraphy with the technetium-99m-labelled anti-granulocyte antibody anti-NCA-95 (BW 250/183, IgG1) was studied retrospectively in 34 consecutive patients with FUO. Every effort was made to confirm a diagnosis, including methods such as ultrasonography, computed tomography, magnetic resonance imaging, bacteriological tests, surgical intervention and clinical follow-up. In 58.8% of the patients, an infectious cause for the fever was found, in 30.2% of the patients, a benign or malignant haematological disease, pancreatitis or thyrotoxicosis was found. No cause for fever could be found in 11%. The overall diagnostic sensitivity and specificity of immunoscintigraphy for infection were 40% and 92% respectively. The positive predictive value was calculated to be 88% and the negative predictive value was calculated to be 52%. False-negative scans were especially noted in patients with endocarditis, pneumonia and small brain abscesses, where the lesions did not exceed a diameter of 0.5 cm. If patients with endocarditis were excluded, the imaging sensitivity and specificity were increased to 57% and 95%. This study demonstrates that 99mTc-anti-NCA-95 scanning is able to localize infectious causes of FUO, other than endocarditis.