In the period 1962-1974, 36 patients with X-linked ichthyosis, belonging to 22 families, were selected for study. The frequency of this genodermatosis is higher in the province of Salamanca than in other countries. In most of the cases, the lesions were apparent at birth or shortly after. In 41% of the patients studied by slit-lamp microscopy, posterior embriotoxom was seen. Hyperkeratosis was seen in follicular and sweat duct orifices. Statistical analysis of the pedigrees showed a higher proportion of affected males among the offspring of carriers and also among the offspring of the ichthyotics' sisters. In addition, among the patients' descendants, the higher ratio of females is statistically significant, showing that there is likely to be selection in favour of the X chromosome.