Healing characteristics and rates of resolution were retrospectively reviewed in 38 bone lesions in 18 patients with histiocytosis X. Signs which tended to indicate future healing were a change from a nontrabecular pattern to a trabecular pattern by 10 weeks, a nonsclerotic lesion to one with sclerosis by 13 weeks, and loss of distinct margins by 24 weeks. Complete healing then occurred within 36 to 40 weeks after development of these favorable signs. The earliest occurrence of a favorable sign was 6 weeks. A trabecular pattern was found in 90% of the lesions, outside the skull, which eventually healed. Fifty-eight percent (22 of 38) of the lesions completely healed in an average of 13.6 months. Five months was the most rapid occurrence of healing in any of our cases.