Hand radiographs of 100 girls representing 73% of the known Australian population of girls with Rett syndrome, age 20 years or less, were available for this study. Control radiographs were matched for age, sex, and laterality. Bone age was assessed against standard radiographs in Greulich and Pyle [1959: Radiographic Atlas of Skeletal Development of the Hands and Wrist, 2nd ed.]. A metacarpophalangeal pattern (MCPP) profile comparing the relative lengths of the hand bones with mean population norms by age was produced by converting the length of each of the 19 metacarpal and phalangeal bones into a Z score. In girls less than 15 years old, bone age was more advanced in Rett syndrome than in age-matched control girls (left hand P = 0.03, right hand 0.004), but was most advanced in the younger group and normalized with age. In Rett syndrome, the mean Z score for the 19 metacarpal and phalangeal bones was 1.0 in children under 5 years, -0.27 in those aged 5-11 years, and -1.7 in those aged 12 years and over. This variation between age groups was much greater than in the controls. The dips in the MCPP profile occurred at MC2 and D1, and the peaks at M5, P5, and M4. An MCPP profile may provide an additional aid to diagnosis in cases of Rett syndrome where all the criteria are not met, but in children under age 5 years, advanced bone age may be more helpful as a marker. Am.