Language development in 32 preschool siblings (aged 2-6 years) of children with diagnosed autistic spectrum disorder (ASD) was compared with that of a control group of 28 typical preschool children. Groups were matched by siblings' age, gender, maternal educational level and family income. The mean ages of the siblings group and the control group were 4.2 and 4.4 years. Eight of the siblings had delayed language development, of whom three received a diagnosis of developmental language disorder (DLD) and one of ASD. The sibling with ASD and two of those with DLD were excluded; the remaining 29 siblings and the controls were administered the Stanford-Binet IV. Verbal IQs of siblings were not significantly different from the control group. Siblings of children with ASD associated with intellectual impairment ("mental retardation" (MR) in Thailand) had significantly lower verbal IQ scores than siblings of children with ASD but without MR. (Contains 3 tables.)