The B cell–depleting monoclonal antibody rituximab is a novel therapy for the rheumatic diseases, with an increasing body of evidence regarding its safety and efficacy in an expanding range of indications. However, there is uncertainty over its potential use in, and impact on, autoantibody-negative diseases. We describe 3 patients, with no known risk factor for psoriasis, who developed psoriasis (and 1 who also developed features of psoriatic arthritis) after receiving rituximab for a variety of indications, namely, seropositive and seronegative rheumatoid arthritis and systemic lupus erythematosus. In all cases, the underlying disease responded well to rituximab. The interpretation of this possible side effect of rituximab remains unclear, but a B cell–depleted environment may induce abnormal T cell responses, possibly provoked either by subclinical infection or by the removal of mechanisms whereby B cells regulate T cells. These cases suggest that the pathogenesis of psoriasis may not require normal numbers of B cells and that proposed treatment of psoriasis and psoriatic arthritis with rituximab may result in unpredictable responses.