The findings of 49 67Ga citrate scans were correlated with all clinical information obtained by other diagnostic modalities in untreated and treated patients with histopathologically proven Hodgkin's disease (539 sites). Eleven nodal and extranodal sites were analyzed in terms of sensitivity, specificity, accuracy and, subsequently, by calculations of the posterior probability for the presence of disease at positive and negative sites by scan using Bayes' theorem. Lowest posterior probabilities for the presence of disease at positive sites were seen in lung parenchyma (66 percent) and spleen (67 percent) in untreated patients and for treated patients in adbominal nodes an inguinal nodes (both 66 percent), whereas the scan was a good method for detecting Hodgkin's disease at all other sites where posterior probabilities were near 100 percent. Figures for the posterior probability of disease at negative sites by scan showed it was a good modality for ruling out Hodgkin's involvement in axillary nodes, perihilar nodes, lung parenchyma, inguinal nodes, and bone marrow (all less than 10 percent); however, it was not as satisfactory in ruling out disease in untreated cervical nodes (20 percent), supraclavicular nodes (11 percent), and especially in the intraabdominal sites of spleen (35 percent), abdominal nodes (33 percent), and periaortic nodes (27 percent).