Autoimmune diseases affect up to 10% of the world's population and, as a whole, they are far more common in females, although differences exist according to the single disease and also in different age groups. In childhood-onset autoimmune diseases, the sex bias is generally less evident than in adults, probably for the different hormonal milieau, being estrogens strongly implicated in the development of autoimmunity. Still, some rheumatic conditions, such as juvenile idiopathic arthritis (JIA), show a strong predilection for girls (F:M = 3-6.6:1), and differences may coexist between males and females regarding disease outcome. For example, chronic anterior uveitis associated with JIA affects more commonly girls but boys tend to have a more severe course. Systemic lupus erythematosus predominantly affects girls and women (F:M = 3-5:1 in children, F:M = 10-15:1 in adults). Behςet's disease has been reported to be more prevalent in adult males (F:M = 1:1-4); in children, there are no differences. The sex ratio is equal in children and adults for Henoch-Schönlein purpura (F:M = 1:1). A higher male-to-female ratio exists for Kawasaki disease (F:M = 1:1.1-1.6 in children, F:M = 1:1,5 in adults). Juvenile dermatomyositis (F:M = 2-5:1), systemic sclerosis (F:M = 4:1 in children, F:M = 6:1 in adults), and Takayasu arteritis (F:M = 2:1 in children, F:M = 7-9:1 in adults) are more common in girls and women then in boys and men. There is no gender bias for acute rheumatic fever in children, while in adults, the F:M ratio is 2:1. Given that estrogen levels are not different between genders during childhood, pediatric rheumatic diseases could represent good models to study other mechanisms related to the development of autoimmunity. Recently, the levels of miRNA expression, and their variation according to sex chromosomes, have been linked to the development of autoimmune diseases, with different impact among sexes. This review will focus not only on the sex bias reported in the more common rheumatic conditions of childhood, focusing on differences in incidence, but also on outcome and trying to depict the mechanisms underlying those differences.