Bone development, casein-free diet use, supplements, and medications were assessed for 75 boys with autism or autism spectrum disorder, ages 4-8 years. Second metacarpal bone cortical thickness (BCT), measured on hand-wrist radiographs, and % deviations in BCT from reference medians were derived. BCT increased with age, but % deviations evidenced a progressive fall-off (p = 0.02): +3.1 [plus or minus] 4.7%, -6.5 [plus or minus] 4.0%, -16.6 [plus or minus] 3.4%, -19.4 [plus or minus] 3.7%, -24.1 [plus or minus] 4.4%, at ages 4-8, respectively, adjusting for height. The 12% of the boys on casein-free diets had an overall % deviation of -18.9 [plus or minus] 3.7%, nearly twice that of boys on minimally restricted or unrestricted diets (-10.5 [plus or minus] 1.3%, p less than 0.04), although even for boys on minimally restricted or unrestricted diets the % deviation was highly significant (p less than 0.001). Our data suggest that the bone development of autistic boys should be monitored as part of routine care, especially if they are on casein-free diets.