We tested whether maternal vitamin D insufficiency during pregnancy is related to the autism phenotype. Serum 25(OH)-vitamin D concentrations of 929 women were measured at 18 weeks' pregnancy. The mothers of the three children with a clinical diagnosis of autism spectrum disorder had 25(OH)-vitamin D concentrations above the population mean. The offspring of 406 women completed the Autism-Spectrum Quotient in early adulthood. Maternal 25(OH)-vitamin D concentrations were unrelated to offspring scores on the majority of scales. However, offspring of mothers with low 25(OH)-vitamin D concentrations (<49 nmol/L) were at increased risk for 'high' scores (≥2SD above mean) on the Attention Switching subscale (odds ratio: 5.46, 95% confidence interval: 1.29, 23.05). The involvement of maternal vitamin D during pregnancy in autism requires continued investigation.