We studied 104 patients with a total of 154 hemivertebrae which had produced scoliotic curves. Of the hemivertebrae 65% were of a fully segmented (non-incarcerated) type, 22% were semi-segmented and 12% were incarcerated. We found that the degree of scoliosis produced depended on four factors: first, the type of the hemivertebra; secondly, its site; thirdly, the number of hemivertebrae and their relationship to each other; and finally, the age of the patient. Semi-segmented and incarcerated hemivertebrae usually do not require treatment. Fully segmented non-incarcerated hemivertebrae may require prophylactic treatment to prevent significant deformity.