In a cooperative study in The Netherlands 22 patients with lissencephaly type I were collected. In the period 1980-1988, the prevalence of lissencephaly type I in the Netherlands was 11.7 per million births (12.2 for females and 11.0 for males). The 5-year survival rate of patients with a complete or nearly complete agyria (grade 1 or 2) in this study was 54% compared to 91% in patients with a mixture of agyria with pachygyria or complete pachygyria (grade 3 or 4).