Following treatment for mediastinal lymphoma, residual masses are defined as a mass greater than 2 cm observed on the CT scan in the absence of other evolutive signs of lymphoma. In this study, we examined 55 patients with residual mediastinal mass after optimal therapy, using gallium scan (37 cases) or magnetic resonance imaging (MRI, 44 cases). The group comprised 41 subjects with Hodgkin's disease and 14 with non Hodgkin's lymphoma, stages I and II (32 cases) and stages III and IV (23 cases); 35 subjects (64%) having bulky mediastinal involvement at diagnosis. A negative gallium scan or fibrotic signal on MRI was correlated with complete remission in 48 cases (87%). In seven of these 48 patients, MRI was not conclusive with a high signal of indeterminate tissue following radiotherapy, but four of the seven had fibrotic tissue on biopsy and none relapsed. Positive gallium scan was observed in six cases. In conclusion, we suggest performing a gallium scan at the end of induction chemotherapy and when it is negative, treatment may be continued without surgical biopsy or salvage therapy. MRI is of value when it shows fibrotic tissue but can be inconclusive.