Two randomized, placebo-controlled, double-blind studies have shown clinical benefit with omalizumab (Xolair), a recombinant humanized monoclonal anti-immunoglobulin E (IgE) antibody, at a dose of 300 mg every 3 or 4 weeks in patients with seasonal allergic rhinitis. The present open-label, 12-week study was designed to assess the safety and tolerability of retreatment with omalizumab in 287 patients previously treated with this agent in one of the latter studies. Omalizumab, 300 mg, was administered subcutaneously every 4 weeks (three injections) to patients with IgE levels < or = 150 IU/mL (n = 182) and every 3 weeks (four injections) to patients with IgE levels > 150 IU/mL (n = 105) at screening before retreatment. Reported adverse events were monitored and blood samples were analyzed for laboratory safety (hematology and serum chemistry) and IgE levels. Urinalysis also was completed as part of the laboratory safety evaluation. The overall incidence and pattern of adverse events were similar to those reported in the primary study. There were no severe or serious adverse events related to omalizumab treatment and no anti-omalizumab antibodies were detected in any patient. Two patients withdrew from treatment because of adverse events (skin rash and nausea; facial erythema and edema) related to study treatment. Free IgE levels decreased to the levels associated with symptom reduction in the core study. In summary, retreatment during a second pollen season with omalizumab, 300 mg every 3 or 4 weeks, was well tolerated and was not associated with any significant immunologic reactions.