The prevalence of type I complement C2 deficiency in Swedish systemic lupus erythematosus (SLE) patients was investigated by DNA analysis. The characteristic 28 base pair deletion was determined by polymerase chain reaction analysis followed by gel electrophoresis. Five of the 86 patients (5.8%) retrieved from a defined population of 160,000 individuals were heterozygous for the C2Q0 gene compared with one heterozygote of 100 local blood donors (1%), the difference in prevalence not being significant. Among 26 other SLE patients, two patients who are siblings were C2Q0 homozygous. No distinctive clinical features among the patients with C2Q0 genes were obvious, although none had renal involvement.