We describe the detailed clinical, pathologic, and biochemical features of brother and sister with the neonatal onset form of adrenoleukodystrophy, together with evidence of the biochemical defect. When compared with reports of previous cases, it becomes clear that this is a newly described clinical entity with remarkable uniformity of signs and very different from the usual childhood form. Some pathologic features are shared, including the morphologic abnormality of the adrenal in both neonatal and childhood forms, but deposition of abnormally metabolized lipids is more systemic and widespread in the neonatal form. The biochemistry of the disease is presented in both children and parents. Plasma values of long-chain fatty acid C26:0 are 0.328 +/- 0.18 micrograms/ml in a control population and 0.381 +/- 0.312 micrograms/ml in the father and mother. Values for C26:0 in the plasma of childhood adrenoleukodystrophy are 1.62 +/- 0.87 micrograms/ml and in our two cases, 2.79 micrograms/ml in the male, 1.83 micrograms/ml in the female. The basic biochemical defect appears to be a diminished capacity to oxidize these fatty acids leading to accumulation in cholesterol esters. Fatty acid oxidation to CO2 by cultured skin fibroblasts was 51% of control value for stearic acid, 5% for lignoceric acid in the male, and 39% of control value for stearic acid, 5% for lignoceric acid in the female. The genetics of this disease is different; whereas childhood adrenoleukodystrophy is X-linked, the neonatal onset form affects males and females equally and is most probably autosomally recessive in inheritance.