OBJECTIVE To determine the predictive value of gallium scans in patients with mediastinal Hodgkin's disease treated with chemotherapy or combined modality treatment. METHODS A retrospective study was performed of 48 patients with mediastinal Hodgkin's disease treated with chemotherapy or combined modality therapy. Patients were monitored with whole-body planar scans (34 patients) or chest single-photon-emission computed tomography (SPECT) plus planar abdominal imaging studies (14 patients). Scans were performed at diagnosis, following three to eight cycles of chemotherapy, and after the end of treatment. The value of gallium scans in modifying treatment and predicting outcome was assessed. RESULTS All patients studied at the time of diagnosis had abnormal gallium accumulation in the mediastinum. After chemotherapy, four patients had residual mediastinal activity; two patients with persistent activity on planar scans failed to enter remission and died of disease; two other patients with abnormal activity only seen on SPECT had therapy modified and remain in remission. After chemotherapy, 44 patients had a normal gallium scan. Twelve patients with negative scans relapsed, including nine patients with recurrence above the diaphragm. CONCLUSIONS The use of gallium scans after several courses of chemotherapy resulted in a modification of treatment in four patients, including two patients who are apparently cured. However, after negative scans, 20% of patients relapsed above the diaphragm. These results suggest that gallium imaging, including SPECT, is of limited value in predicting disease sterilization, although the number of patients studied with SPECT was small. At present, the major value of gallium scans is to identify patients who may benefit from a modification of treatment.