Objective. —We review the clinical phenotype, pathological changes, and results of cytogenetic and molecular genetic studies in 90 probands with lissencephaly (smooth brain) with emphasis on patients with the classical form (type I). We also describe the recent discovery of the lissencephaly gene (LIS1), deletions of which have been implicated as the cause of this disorder in many patients. Data Sources. —We have performed clinical, cytogenetic, and molecular genetic studies of 25 probands with Miller-Dieker syndrome and 65 probands with isolated lissencephaly sequence (ILS). We have further subdivided patients with ILS into those with classical lissencephaly and those with lissencephaly variants. Study Selection. —We consider primarily our own published and unpublished data, but include references to studies of other series of patients with lissencephaly. Data Synthesis. —Visible cytogenetic deletions of 17p13.3 were detected in 14 of 25 Miller-Dieker syndrome probands, and either visible cytogenetic or submicroscopic deletions in 23 (92%) of 25. Submicroscopic deletions were detected in eight of 45 patients with all types of ILS. If only ILS patients with the classical form are considered, we detected deletions in eight (38%) of 21. Conclusions. —Deletions of the lissencephaly critical region in chromosome 17p13.3, includingLIS1, appear to be the most frequent cause of classical lissencephaly. Molecular cytogenetic studies, particularly fluorescence in situ hybridization, should be performed in all such patients.LIS1shows homology to genes involved in signal transduction, which may be its function in development of the telencephalon. Other genetic causes of classical lissencephaly and genetic and nongenetic causes of other types of lissencephaly exist and are under study. (JAMA. 1993;270:2838-2842)