The wide phenotypical heterogeneity within the ichthyosis congenita group of diseases is well known. We report a case of a very rare and unusual autosomal recessive ichthyosis congenita, type IV, according to the ultrastructural classification. Our case presented the triad clue for the diagnosis, characterized by follicular hyperkeratosis, prematurity and perinatal complications, but the clinical diagnosis was further complicated by hypereosinophilia and a strongly positive Darier's sign suggesting diffuse cutaneous mastocytosis. The diagnosis was provided only by electron microscopy, which showed the pathognomonic markers of ichthyosis congenita type IV, namely a large number of membrane structures in the stratum corneum and stratum granulosum. As a consequence, correct genetic counselling for the parents was carried out, and they were informed about the benign course of the disease after the complications of the perinatal period. This case is a further example of the reliability of ultrastructural markers in the diagnosis of inherited keratinization disorders, especially those with an unusual clinical appearance.