Nonbullous congenital ichthyosiform erythroderma (CIE) and classic lamellar ichthyosis (LI) can be distinguished by clinical, histopathologic, and biochemical findings reported herein. Whereas all patients with CIE (n = 12) had fine white scales, they were heterogeneous with respect to degree of erythroderma, ectropion, and prognosis. In contrast, all patients with LI (n = 6) had large, dark, platelike scales, severe ectropion that showed no improvement with age, and minimal erythroderma. Although the stratum corneum of patients with LI was two to three times thicker than that of those with CIE, the latter group demonstrated more acanthosis, parakeratosis, hypergranulosis, and less prominent dermal capillaries. Studies of scale-lipid content have indicated biochemical correlates of this clinical heterogeneity. These clinical, histologic, and biochemical findings provide useful guidelines to differentiate CIE from LI and strongly suggest that autosomal recessive primary ichthyosis comprises two distinct diseases.