Deficiency in the second component of complement (C2) is the most common homozygous complement deficiency. While approximately half of the affected individuals are apparently healthy, C2 deficiency may be associated with autoimmune diseases and rarely increased susceptibility to infection. We report 5 patients who had homozygous type I C2 deficiency in two families. Three of them suffered from frequent infections. These symptomatic patients had additional risk factors; the index cases in the first and the second family had IgG2 deficiency and IgA deficiency, respectively, and alternative complement pathway hemolytic activity was also low in both of them and in the sibling of the first index case. These results emphasize the probable role of other immunologic defects in the clinical presentation of C2 deficiency.