BACKGROUND Gallium-67 (Ga-67) is widely used in the assessment of patients with lymphoma. Although highly sensitive in Hodgkin's disease and high grade non-Hodgkin's lymphoma (NHL), its sensitivity is reported to be reduced in lower grades of NHL. Several series have suggested that thallium-201 (Tl-201) may be more useful than Ga-67 in the evaluation of low and possibly intermediate grade NHL. OBJECTIVE To compare the sensitivity and relative avidities (uptakes) of Tl-201 and Ga-67 in patients with NHL. METHODS A total of 36 pairs of studies were performed in 33 patients (14 male: 19 female; age range 16-84 years, mean 55 years). All had whole body Ga-67 (400 MBq) and Tl-201 (100-200 MBq) studies (interval 0-three days) which were read by two experienced nuclear medicine physicians blinded to clinical history and results of other investigations. RESULTS Tl-201 had a higher sensitivity and avidity than Ga-67 in patients with low grade NHL. In contrast, Ga-67 was slightly more sensitive and had a higher avidity in patients with both intermediate and high grade NHL. CONCLUSIONS We conclude that Tl-201 is more sensitive and avid for low grade NHL than Ga-67 and that consideration should be given to its use in this condition in conjunction with Ga-67 as it may provide complementary information. Routine use in intermediate grade NHL is less convincing as Ga-67 was superior in most cases. Ga-67 is more sensitive than Tl-201 in assessment of high grade NHL.