Since 1973, we have identified and collected follow-up data on 16 patients with hypocomplementemic urticarial vasculitis. Preliminary diagnostic criteria are the presence of typical urticarial skin lesions and low levels of serum complement (all components), plus two of the following: dermal venulitis, arthritis, glomerulo-nephritis, episcleritis or uveitis, recurrent abdominal pain, and C1q precipitin in plasma. Exclusions are systemic lupus erythematosus, mixed cryoglobulinemia, elevated antinuclear antibody titer, hereditary deficiency of a complement component or of C1 esterase inhibitor, and presence of anti-native DNA or hepatitis B antigen. The renal involvement is relatively benign, and generally the patients do well and respond to specific treatment when this is indicated. Eight of 10 smokers studied had evidence of chronic obstructive pulmonary disease, 1 of whom died of this complication. In three patients, severe chronic obstructive pulmonary disease developed at a young age after relatively low pack-year cigarette smoking histories. Lung disease probably results from the interaction of two major risk factors-smoking and an immunologically mediated process that has not been identified.