Previously considered to represent a single genetic disorder, autosomal recessive ichthyosis was examined in clinical and lipid biochemical studies of 18 patients with this condition and instead disclosed to be two distinct diseases. Six patients displayed clinical features of classical lamellar ichthyosis (LI), which is characterized by monomorphous features, including large, dark, platelike scales, severe ectropion, and a uniformly severe, unremitting course. 11 patients displayed clinical features of nonbullous congenital ichthyosiform erythroderma (CIE) characterized by fine white scales, prominent erythroderma, a milder course, and a variable prognosis. CIE could be separated biochemically from LI by the invariable presence of elevated quantities of n-alkanes in scale (CIE, 24.8 +/- 1.9% vs. LI, 7.2 +/- 1.6%, and normal, 6.5 +/- 0.9%), which suggested a primary disorder in neutral lipid metabolism. In light of the distinctive clinical features of each, these biochemical studies indicate that autosomal recessive ichthyosis comprises two distinct disorders.