The aim of this work was to perform an immunological study in six patients with 21 hydroxylase deficiency in mild form (M210HD) and in 2 patients with 21-hydroxylase deficiency in classical form (C210HD) and in their parents, in whom a previous HLA,C4,Bf typing demonstrated high prevalence of DR5 and phenotypic absence of fraction C4B of complement (C4BQO). This study contains the evaluation of C3, IgA, IgG, IgM levels, anticardiolipin antibodies (IgG and IgM) and circulating immunocomplexes. A study of lymphocyte subsets was also performed. Among M210HD 1 patient showed presence of anticardiolipin antibodies both IgM and IgG; this patient had shown antinuclear antibodies in a previous study. Among parents, some subjects showed presence of anticardiolipin antibodies and high levels of circulating immunocomplexes. No alterations in C3 and Ig levels were observed. A reduced percentage of CD4 suppressor-inducer (CD4-SI) (p < 0.05 in M210HD and in parents vs controls) and increased percentage of CD4 helper-inducer (CD4-HI) (p < 0.05 in both groups vs controls) were found. No alterations were evidenced in C210HD patients. Data about association between 21-hydroxylase deficiency and autoimmune diseases are rare. Our results confirm that 210HD could be associated to an unbalancement of immune system function and suggest that non immune genes, like 21-hydroxylase one, may influence the expression of autoimmune diseases at least in presence of peculial extended haplotypes.