We report results of clinical, cytogenetic, and molecular studies in 27 patients with Miller-Dieker syndrome (MDS) from 25 families. All had severe type I lissencephaly with grossly normal cerebellum and a distinctive facial appearance consisting of prominent forehead, bitemporal hollowing, short nose with upturned nares, protuberant upper lip, thin vermilion border, and small jaw. Several other abnormalities, especially growth deficiency, were frequent but not constant. Chromosome analysis showed deletion of band 17p13 in 14 of 25 MDS probands. RFLP and somatic cell hybrid studies using probes from the 17p13.3 region including pYNZ22 (D17S5), pYNH37 (D17S28), and p144-D6 (D17S34) detected deletions in 19 of 25 probands tested including seven in whom chromosome analysis was normal. When the cytogenetic and molecular data are combined, deletions were detected in 21 of 25 probands. Parental origin of de novo deletions was determined in 11 patients. Paternal origin occurred in seven and maternal origin in four. Our demonstration of cytogenetic or molecular deletions in 21 of 25 MDS probands proves that deletion of a "critical region" comprising two or more genetic loci within band 17p13.3 is the cause of the MDS phenotype. We suspect that the remaining patients have smaller deletions involving the proposed critical region which are not detected with currently available probes.