We report on the results of an array comparative genomic hybridization (array CGH) study of 150 karyotypically normal Finnish patients with idiopathic mental retardation and/or dysmorphic features and/or malformations. Using high-resolution microarray analysis, we sought to identify clinically relevant microdeletions and microduplications in these patients. The results were confirmed using other methods and compared with findings reported in recent publications and internet databases. Small aberrations of potential clinical significance were found in 28 (18.6%) of the 150 patients. Eight of the identified aberrations are known to cause syndromes, 4 affected the X chromosome in males, 4 were familial, and 13 have yet to be associated with a phenotype. This study demonstrates the benefits of array CGH in clinical diagnostics of developmental disorders. Further, our findings give evidence of new syndromes.