Systemic lupus erythematosus (SLE) shows various clinical manifestations, which are characterized by inflammation in many different organ systems. The cause of SLE is still unclear; however, the immunological abnormalities are considered to be responsible for the pathogenesis of SLE. As angiotensin I-converting enzyme (ACE) has been reported to be associated with various immunological phenomena, we investigated the correlation between insertion (I)/deletion (D) polymorphism of the ACE gene and the disease activity of SLE. Ninety-three patients with newly diagnosed SLE were enrolled in this study. ACE genotype was determined by the polymerase chain reaction (PCR). We measured serum levels of anti-double-stranded (ds) DNA antibody (Ab) and serum levels of total complements (CH50) as the parameter for lupus activity. Moreover, we evaluated the clinical disease activity by calculating SLE disease activity index (SLEDAI). Individuals with II genotype showed a significant increase in SLE activity. Patients with the ACE II genotype showed a higher serum level of anti-dsDNA Ab (14.3 IU/ml (5.475, 74.6, median (25th centile, 75th centile)) than those with the DD genotype (4.65IU/ml (4.05, 6.8)) (P<0.01). Moreover, patients with the 11 genotype also showed lower levels of serum CH50 than those with the DD genotype (P < 0.01). Patients with the II1 or DI genotype had significantly higher SLEDAI score than those with the DD genotype (P < 0.01). These results suggest that the ACE genotype could be associated with the disease activity of SLE. ACE insertion polymorphism might be used as one of predictive factors for the activity of lupus.