To evaluate the immunogenicity, safety, and efficacy of 50 mg/mL liquid etanercept. In a multicenter, open-label study, adults with active rheumatoid arthritis (RA) received 50 mg/mL liquid etanercept subcutaneously once weekly for 24 weeks. Immunogenicity was assessed at baseline and weeks 24 and 28, safety at all study visits, and efficacy at baseline and weeks 12 and 24. Of 222 treated patients, 88% completed the study; 81% were women; 84% were white; mean age was 53 years; mean RA duration was 10 years. Antibodies to etanercept, all non-neutralizing, were detected in 12 of 214 patients; 7 of the 12 were borderline positive (antibody titers <1:50). The presence of non-neutralizing anti-etanercept antibodies did not appear to affect clinical safety or efficacy. Few patients reported serious adverse events (6.3%), serious infections (2.3%), or withdrew because of adverse events (4.5%). Most adverse events were mild or moderate. The most common event, injection site reaction, occurred in 29.3% patients. At week 24, 63% of patients achieved an ACR20 response, 36% an ACR50 response, and 14% an ACR70 response. Similar responses were apparent by week 12. Week 24 mean improvement in the Health Assessment Questionnaire disability index scores was 0.6 points; improvement in the Short Form-36 Physical Component Score was 10.0 points. The 50 mg/mL liquid etanercept formulation administered once weekly was well tolerated. The incidence of anti-etanercept antibodies, the nature and frequency of adverse events, and improvements in signs and symptoms of RA and patient physical function were similar to those in previous etanercept studies.