We report seven new patients with clinical features of the Smith-Magenis syndrome (SMS) and small de novo interstitial deletions of 17p11.2. Four of these patients had been referred for fragile-X studies, but standard G-banded chromosome analysis routinely carried out in addition to the fragility tests revealed the microdeletion in chromosome 17. A relatively high proportion (approximately 2%) of patients referred to this Centre for fragile-X investigation are found to have a chromosome abnormality other than fra(X)(q27.3), half of them (approximately 1%) with an unbalanced chromosome complement. The four of our seven patients with deletion 17p11.2 constitute 25% of those with an unbalanced karyotype, and establish this microdeletion as the most common chromosome abnormality-except for fra(X)(q27.3)-in patients referred for fragile-X screening. The data indicate that standard karyotyping should be offered to patients with this referral indication, in addition to any molecular or chromosome fragility tests for fragile X. We also recommend that the short arm of chromosome 17 be examined critically in these patients. Moderate quality and resolution of banding (450-550 bands per haploid chromosome set) are adequate for detection of the 17p11.2 deletion.