The prevalence of anti-endothelial cell antibodies (AECA) of IgA, IgG and IgM classes was studied by means of enzyme-linked immunosorbent assays (ELISA) in 466 patients with autoimmune/inflammatory disorders. The reference limits in the ELISAs for the AECA were determined from a random population sample of 249 subjects. The frequency of AECA was highest in patients with SLE (n = 42), 14.6% mainly of IgG class, and the presence of AECA correlated with disease activity in these patients. In the RA patient group (n = 200), 9.5% had AECA, mostly of IgA type. We found no association between the presence of AECA and extra-articular manifestations of RA or survival rate. In patients with undefined connective tissue disease (n = 57), ankylosing spondylitis (n = 109), and psoriatic arthritis (n = 58), the frequency of AECA corresponded to that of the random population sample. In a cohort of samples sent to the laboratory for determination of anti-nuclear antibodies (ANA) there was a correlation between the presence of ANA and AECA. Our findings indicate that RA patients are characterized by IgA class AECA, whereas SLE patients have IgG class AECA also correlating to disease activity.