Various immunologic abnormalities have been implicated in cold urticaria. This is the first report of cold urticaria associated with C4 deficiency and elevated IgM. A 12-year-old male developed urticaria upon exposure to cold. He denied fever, purpura, hemoglobinuria, Raynaud's disease, or arthralgias. Family history was negative for cold urticaria. Immunologic studies revealed elevated IgM (186 mg/dL) as well as decreased CH100 and C4 (8.0 mg/dL). C1, C2, and C3 were normal. Ice cube skin test was positive, but passive transfer tests were negative. Biopsy was not diagnostic for vasculitis, although it revealed a few immunofluorescent deposits of IgM and C4. Complement genetic studies revealed deficiency of two half-null C4 haplotypes expressed as C4A*3QO and B*2QO.