The results of planar and single photon emission computed tomography (SPECT) gallium-67 scintigraphy performed before and after treatment were compared in 77 patients with lymphoma at 240 sites. Before treatment, the sensitivity of planar scintigraphy was 78% and the specificity was 97%; after treatment, 84% and 96%, respectively. Before treatment, SPECT had a sensitivity of 85% and a specificity of 98%; after treatment, 92% and 99%, respectively. In addition, SPECT allowed better anatomical localization and showed the entire extent of lesions at 36 sites. SPECT was of special value after treatment in excluding disease at 16 sites with residual masses seen at computed tomography (CT). Long-term follow-up in these patients showed that their condition was in complete remission and that the CT appearance of a residual mass does not always mean residual cancer. Thus, Ga-67 SPECT is a suitable imaging technique for monitoring the response of lymphoma to treatment.