Recent reports have revealed the presence of subtelomeric rearrangements in 0.5-1.1% of patients with mild mental retardation and in 6.8-7.4% of patients with moderate-severe mental retardation. In the present study, 130 patients with unexplained mental retardation were tested using fluorescence in situ hybridization (FISH) analysis for the first time in a large group of Turkish patients, in order to determine the frequency of subtelomeric rearrangements. Three patients bad such rearrangements. We present the clinical findings in these patients with (1) coexistent 9p subtelomeric monosomy and 4q subtelomeric trisomy, (2) 22q13.3 subtelomeric monosomy, and (3) coexistent 4p subtelomeric monosomy and 8p subtelomeric trisomy. Mild retardation without dysmorphic features in one of these patients suggests offering subtelomeric analysis to a wide spectrum of mental retardation.