Aim: To study the sex ratio (proportion of males) in siblings of individuals diagnosed with autism spectrum disorders (ASDs) as children. Method: In the current study, we extended previous studies dealing with the androgen theory of autism and examined sex ratios in the siblings of 326 individuals with ASD (245 males, 81 females) who had been consecutively assessed at two Danish university clinics of child psychiatry during the 25-year period from 1960 to 1985. Results: Among the 513 siblings, 300 were males and 213 females. This yields a sex ratio of 0.585, which is significantly higher than the Danish live-birth sex ratio over the same period (0.514, p=0.001). The sibling sex ratio was not associated with the IQ in the autistic probands. Interpretation: Our findings suggest a potential indirect confirmation of the androgen theory of autism.