Rituximab, a human/mouse chimeric anti-CD20 antibody, has become part of standard therapy for patients with CD20-expressing B-cell lymphoma, and is currently under investigation for other indications including autoimmune diseases, in particular rheumatoid arthritis (RA). Its characteristic tolerability profile was established soon after clinical testing began and compares favourably with chemotherapy. The majority of patients experience mild to moderate infusion-related reactions (IRRs) during the first administration of rituximab, but the incidence decreases markedly with subsequent infusions. Current data suggest that the type of adverse events in patients with RA are similar to those in lymphoma, but that adverse events related to the rituximab infusions are less severe and less frequent. Rituximab induces a rapid depletion of normal CD20-expressing B-cells in the peripheral blood, and levels remain low or undetectable for 2-6 months before returning to pretreatment levels, generally within 12 months. Serum immunoglobulin levels remain largely stable, although a reduction in IgM has been described. T-cells are unaffected by rituximab and consequently opportunistic infections rarely occur in association with rituximab therapy. When used in combination with a variety of chemotherapeutic regimens, rituximab does not add to the toxicity of chemotherapy, with the exception of a higher rate of neutropenia. However, this does not translate into a higher infection rate. Over 540,000 patients worldwide have now received rituximab and serious adverse reactions have occurred in a small minority of patients, but for the great majority of patients, rituximab is safe and well tolerated.