Prospectively, we compared the results of chest radiology and functional imaging, using mIn-labelled polyclonal human IgG (In-HIG), in the identification of pulmonary infection in patients infected by the human immunodeficiency virus (HIV). Sixty-three studies were performed on 57 HIV-infected patients presenting with suspected chest infection or fever of unknown cause, in each of whom a planar chest radiograph was obtained within 24 h of the nlIn-HIG study. The results of the two imaging modalities were compared with the final microbiological or cytological diagnosis. Forty patients were found to have pulmonary infection, 25 of whom were correctly identified with chest radiology (sensitivity 62%) and 39 with lnIn-HIG (sensitivity 97%). In those patients without infection, chest radiology was abnormal in 13 cases (specificity 43%), while there was only one false-positive n,In-HIG study (specificity 95%). lnIn-HIG correctly identified the presence or absence of active lung infection in 61 of 63 cases (accuracy 93%). This was significantly better (x2= 8.25, v = 1, P < 0.01) than chest radiology, which correctly identified the presence or absence of infection in 35 of 63 cases (accuracy 55%). In HIV antibody-positive patients, functional imaging with mIn-HIG is more accurate than chest X-ray in the identification of pulmonary infection. © 1995 Chapman and Hall Ltd.