alpha 1-Antitrypsin levels were determined for 96 patients with various forms of biopsy-proved panniculitis. Fifteen of the 96 patients had alpha 1-antitrypsin deficiency, and 12 of these also had an abnormal alpha 1-antitrypsin phenotype. This group of patients showed substantial and characteristic clinical and histopathologic differences from the group of patients with normal levels and phenotypes of alpha 1-antitrypsin. Spontaneous ulceration and drainage of panniculitis lesions were much more common in patients with alpha 1-antitrypsin deficiency, and biopsy specimens from lesions of panniculitis in these patients were much more likely to show large areas of normal fat adjacent to necrotic lobular and septal areas, which contained many polymorphonuclear leukocytes and histiocytes. Destruction of elastic tissue was more frequent and extensive in patients with alpha 1-antitrypsin deficiency.