Cytogenetic imbalances are the most frequently identified cause of developmental delay or mental retardation, which affect 1-3% of children and are often seen in conjunction with growth retardation, dysmorphic features, and various congenital anomalies. A substantial number of patients with developmental delay or mental retardation are predicted to have cytogenetic imbalances, but conventional methods for identifying these imbalances yield positive results in only a small fraction of these patients. We used microarray-based comparative genomic hybridization (aCGH) to study a panel of 20 patients predicted to have chromosomal aberrations based on clinical presentation of developmental delay or mental retardation, growth delay, dysmorphic features, and/or congenital anomalies. Previous G-banded karyotypes and fluorescence in situ hybridization results were normal for all of these patients. Using both oligonucleotide-based and bacterial artificial chromosome (BAC)-based arrays on the same panel of patients, we identified 10 unique deletions and duplications ranging in size from 280 kb to 8.3 Mb. The whole-genome oligonucleotide arrays identified nearly twice as many imbalances as did the lower-resolution whole-genome BAC arrays. This has implications for using aCGH in a clinical setting. Analysis of parental DNA samples indicated that most of the imbalances had occurred de novo. Moreover, seven of the 10 imbalances represented novel disorders, adding to an increasing number of conditions caused by large-scale deletions or duplications. These results underscore the strength of high-resolution genomic arrays in diagnosing cases of unknown genetic etiology and suggest that contiguous genomic alterations are the underlying pathogenic cause of a significant number of cases of developmental delay.