Self-injury by the host's immune system is believed to be a factor in the etiology of many diseases. Much attention has been given to the part played by sex hormones in such processes. Current literature frequently maintains that females are more susceptible than males to autoimmune diseases. In order to gain information about this factor, a tabulation has been made of the sex incidence of 40 autoimmune diseases which occur at different periods of life: childhood, early adult life, mature adult life, and old age. To some extent the tabulations substantiate female preponderance, but in some there is no gender difference, and in others, particularly the autoimmune nephropathies, male preponderance is the rule. Findings in experiments with animal analogues of human autoimmune disease, showing that administration of estrogen augments the manifestations, whereas androgen treatment suppresses them, do not correlate closely with clinical experience.