The clinical activity of rituximab has been evaluated in a phase 2 study in both untreated and relapsed mucosa-associated lymphoid tissue (MALT) lymphomas. Treatment consisted of 4 standard (375 mg/m2) weekly doses. Thirty-five patients were enrolled, and 34 completed the treatment program. The primary lymphoma location was stomach in 15 patients, and extragastric in 20. Eleven patients had previously been treated with chemotherapy. At study entry 12 patients had Ann Arbor stage IE, 3 had stage IIE, and 20 had stage IV disease. The overall response rate was 73% (95% confidence interval, 56%-87%), with 15 complete responses and 10 partial responses, and the response rate was significantly higher in the chemotherapy-naive patients, who had an 87% response rate compared with 45% of the previously treated patients (P =.03). The median response duration was 10.5 months. At a median follow-up of 15 months, 9 patients (26%) relapsed. The median time to treatment failure was 14.2 months in the whole series, but it was significantly longer (22 versus 12 months) in the chemotherapy-naive patients compared with those who had prior chemotherapy (P =.001). Most adverse events were of mild to moderate severity with no grade 4 toxicity. This study indicates that rituximab is safe with significant activity in MALT lymphomas.