Smith-Magenis syndrome (SMS) comprises a complex physical and behavioral phenotype that is associated with an interstitial deletion of chromosome 17p11.2. The deletions observed in patients can range from <2 to >9 megabases of DNA and may include more than 100 genes. In order to determine the critical deletion interval responsible for the syndrome phenotype, we have examined several patients with varying deletions involving 17p11.2 by somatic cell hybrid analyses. We have binned 112 markers along 17p11.2, including 27 markers within the critical interval for SMS, which is bound proximally by D17S29 and distally by cCI17-638. In addition, we present two patients who carry deletions involving 17p11.2 but do not exhibit the typical features of SMS. Patients such as these will allow genotype:phenotype correlations to be made and the gene(s) responsible for the SMS phenotype to be determined.