An abnormality which has received little attention but may be easily recognized on radiographs of the hand of patients with Turner's syndrome is described. Eleven of thirty-one patients (35.5%) with Turner's syndrome were shown on radiographs of the hand to have a visually detectable smallness of the bone surface area of the carpus when compared to the area of the second through fifth metacarpals. Values for the "C/M" ratio (the area of the carpals divided by the area of the second through fifth metacarpals) were calculated for films of 31 individuals with gonadal dysgenesis and compared with those from bone age-matched films of seventy-six individuals with normal development of the hand and wrist. A consistent difference with minimal overlap was documented. For all of the films of patients with Turner's syndrome the C/M ratio averaged 89% of the value in the films of the controls. In the 11 patients in whose films smallness was visually apparent, the C/M ratio averaged 82% of controls while in those where the diminution was detected only by planimetry the C/M ratio averaged 91% of controls. The incidence of a diminished C/M ratio was compared with the incidence of four other previously described signs of Turner's syndrome; although not seen as often as the coarse reticular pattern or delayed bone age, the C/M sign was recognizable without planimetry at least as commonly as the "metacarpal sign," and a decreased carpal angle and/or Madelung's deformity. Taken together, these several criteria suggest the radiographic diagnosis of Turner's syndrome in more than 93% of cases.