Four patients with the acquired immunodeficiency syndrome (AIDS) and fever were investigated using donor leucocyte scans. The lung/liver and lung/spleen uptake ratios in these patients were compared with the uptake ratios in donor leucocyte scans in seven neutropenic (non-AIDS) patients and five patients who had autologous leucocyte scans performed over the same time period. For all scans indium-111-oxime-labelled leucocytes were used, except for one AIDS patient in whom technetium-99m hexamethyl-propylene amine oxide (HMPAD)-labelled donor leucocytes were used. There were no adverse reactions to the donor cell infusions. Two patients had repeat studies 8 weeks apart (from different donors) without ill effect. There were no differences in the 111In uptake ratios between the three groups. There were three positive studies in the patients with AIDS, and these elucidated the cause of the pyrexia in all three. The negative case is more difficult to confirm, but the clinical course and the absence of focal disease on post-mortem have been taken to support the scan findings. There was no difference in the acceptability of the technique or the distribution of the labelled leucocytes between the AIDS and non-AIDS patients. Donor leucocyte imaging of patients with AIDS is probably more effective and considerably less hazardous for technical staff than autologous leucocyte methods. This study demonstrates that the technique can be applied successfully to patients with AIDS.