Chlamydia trachomatis oculogenital infection is a common disease in western societies. Despite the fact that diabetes is accompanied by increased risk for infections, no data on chlamydial infections in the non-insulin-dependent diabetic (NIDDM) patient exist. In our study Chlamydia antibodies were determined using an immunoperoxidase reaction in NIDDM patients (n = 79) and in a local nondiabetic control population (n = 125) which was randomly invited to a medical control visit without any preselection criteria. In total, 46% of diabetics and 55% of controls were IgG-Chlamydia antibody positive (ns). Using IgA-Chlamydia antibodies to define 'seroactive' chlamydial infection, 22% of NIDDM patients and 14% of controls were positive. Thus seroactive chlamydial infection of all patients with proven contact to Chlamydia (IgG-Chlamydia antibody positive) was 47% in diabetics versus 25% in controls, respectively (P < 0.05). Forming subgroups, significance was reached in females (52% vs. 32%, P < 0.05) only, but a similar trend was observed in males (36% vs. 21%, ns). Seroactivity was neither correlated with HbA1c nor with nephelometrically determined total serum immunoglobulins (IgG, IgA). Additionally we observed significantly elevated total IgM and IgA-levels in NIDDM patients whereas IgG-levels were comparable in both groups. In conclusion, seroactive chlamydial infections in subjects with proven contact to Chlamydia are more frequent in NIDDM patients than in nondiabetic controls. Additionally, higher IgM and IgA serum levels might indicate a higher susceptibility to active surface infections in NIDDM.