There is a distinct group among the patients with unexplained infertility who are found to have enhanced humoral immune response. Recent studies focus on the expression of stress proteins being an important factor in the stages of gametogenesis, fertilization, implantation, early embryonic development and pregnancy. Increased expression of stress proteins and immune response against them was found in tissues exposed to stress. There is not enough data linking infertility to expression and immunity of a-crystallins in patients with endocrine diseases. The aim of this work was to study anti-a-crystallin antibodies in patients with polycystic ovary syndrome (PCOS), Graves' disease, autoimmune thyroiditis, diabetes mellitus and obesity. Sera samples from 169 women with endocrine disorders (PCOS (n = 68); Graves' disease (n = 26); autoimmune thyroiditis (n = 32); diabetes mellitus (n = 10); and obesity (n = 33)) were tested by ELISA. The statistical analysis was performed using SPSS program. The concentration of anti-a-crystallin antibodies is significantly elevated in the PCOS group compared to the control group (p = 0.021). The frequency of positive sera in the same group of patients is significantly higher compared to the control group (p = 0.029). In all other groups, no statistically significant elevation was observed. Elevated concentrations of anti-a-crystallin antibodies found in patients with PCOS suggest that the increased production of anti-a-crystallin antibodies in women with PCOS is most probably caused by failure of the immune tolerance and the induction of immune response. This is most likely due to an increased expression of this stress protein as a result of oxidative stress and chronic inflammation.