The clinical and histologic distinction between X-linked recessive and autosomal dominant ichthyosis was studied by evaluating 12 classical differential parameters in 85 patients. Thirty-three of them had X-linked and 52 autosomal dominant ichthyosis. Eight of these parameters were generally helpful in the differential diagnosis: age of onset, severity of involvement, scale size, chapping of hands and feet, atopic background, influence of warm weather, corneal opacities and state of the granular layer. Involvement of skin folds, keratosis pilaris, increased palmo-plantar markings and improvement with age were unreliable. In the literature, age of onset and corneal opacities were additionally found unreliable; the histology was of limited value in two reports. Therefore, we concluded that the herein evaluated differential criteria seem to be valid mainly when considering groups of patients. For the individual case, an error in diagnosis, particularly in X-linked ichthyosis, is not rare when relying solely on these criteria. When in doubt, determination of steroid sulphatase activity is mandatory.