Liarozole, a novel imidazole derivative, inhibits the cytochrome P450-dependent 4-hydroxylation of retinoic acid, resulting in increased tissue levels of retinoic acid. Twelve male patients with ichthyosis were given oral liarozole, 150 mg twice daily, in an open study for 12 weeks. Immunohistochemical parameters of inflammation, epidermal proliferation and differentiation were assessed before and after treatment. Extent and severity of the skin lesions was markedly reduced in all patients. Clinical side-effects were reminiscent of those with synthetic retinoids. No relevant changes were found in the haematological, urinary and biochemical parameters. Immunohistochemical assessment showed a statistically significant induction of keratin 4 after liarozole treatment in 10 of 12 patients. In two of these patients keratin 13 was induced. This open study showed that oral liarozole treatment was efficacious and well tolerated in the treatment of different types of ichthyosis. The immunohistochemical results suggest a retinoid mechanism as the mode of action.