PURPOSE: Previous research has indicated that children with autism exhibit accelerated head growth (HG) in infancy, although the timing of acceleration varies between studies. We examined infant HG trajectory as a candidate autism endophenotype by studying sibling pairs. METHODS: We retrospectively obtained serial head orbitofrontal circumference measurements of: a) 48 sibling pairs in which one (n=28) or both (n=20) sibs were affected by an autism spectrum disorder (ASD); and b) 85 control male sibling pairs. RESULTS: Rate of HG of ASD subjects was slightly accelerated compared to controls, but the magnitude of difference was below the limit of reliability of standard measurement methods. Sibling intra class correlation for rate of HG was highly statistically significant; the magnitude was significantly stronger among autism-affected families (ICC=.63) than among controls (ICC=.26), p<.01. CONCLUSION: Infant HG trajectory appears familial-possibly endophenotypic-but was not a reliable marker of autism risk among siblings of ASD probands in this sample.