The manner in which typically developing (TD) children were informed about their sibling's diagnosis of autism spectrum disorders (ASDs) was examined in Japan. Seventy-seven parents, each with a child with ASD and a TD child, participated in a questionnaire survey. From the data obtained, it was revealed that parents informed 66.7% (72/108) TD children about their sibling's diagnosis, and usually informed them when they were around 9 years of age. Birth order, age spacing and type of ASD affected the parents' decision about whether or not to inform them, and type of ASD, sex, age spacing and total number of children affected the timing of when parents informed them. TD children whose sibling had high-functioning ASD were informed less often and later. The most common question asked by TD children concerned communication problems, and parents often informed them of symptoms and difficulties of ASD as well as the diagnosis. Before receiving an explanation, 77.9% (53/68) of TD children had guessed the diagnosis or noticed something different about their sibling with ASD and 77.3% (51/66) of them reacted moderately to the parent's explanation. The influence of the parent's explanation on TD children's understanding of ASD warrants further research. (Contains 5 tables.)