Anthropometric development and growth were assessed in 2 groups of 6- to 9-year-olds: children with autism spectrum disorders and typically developing children. In a case-control study conducted in Valencia, Spain, we compared the body mass index (kg/m(2)) of 40 children with autism spectrum disorders (cases) and 113 typically developing children (controls) from the same area of residence. The sex- and age-adjusted odds ratios for being underweight in cases was 2.41 compared to controls. Furthermore, the body mass index distribution of the cases was significantly offset to lower values with respect to that of the controls (P = .024). In particular, 20% of the cases had a body mass index below the fifth percentile versus just 8.85% of the controls. Our data suggest that the anthropometric development of children with autism spectrum disorders should be monitored as part of routine care.