Mental retardation (MR) has a 2-3% frequency in general population. Several studies have reported that unbalanced cryptic subtelomeric rearrangements are a significant cause of idiopathic MR cases who have had apparently normal karyotypes with GTG-banding. Most of the telomeric regions are G-band negative and morphologically similar to each other. Therefore, the cryptic rearrangements of these regions have been proved by Fluorescence In Situ Hybridization (FISH) using chromosome-specific subtelomeric probes. In our study, 85 cases with idiopathic MR and dysmorphic features with normal conventional karyotypes were investigated by FISH using subtelomeric region specific probes. Subtelomeric chromosomal rearrangaments were detected in 10 of 85 patients as 46,XX.ish del(1)(pter)de novo in one patient, 46,XX.ish del(2)(qter)de novo in one patient, 46, XX.ish del(4)(pter) de novo in two patients, 46,XX.ish del(9)(pter) de novo in one patient, 46, XX.ish del(9)(qter) in one patient, 46,XX.ish der(5)t(5;15)(pter;qter)pat in two brothers, 46,XY.ish der(10)t(10;18)(pter;pter) de novo in one patient, 46,XX.ish rec(10)dup(10pter)-inv(10)(pterqter)mat in one patient, 46,XX.ish der(18)t(18;22)(qter;qter) de novo in one patient. Except for one case, the parents of all cases with cryptic subtelomeric chromosomal rearrangements were also evaluated by FISH. In conclusion, the detection of subtelomeric chromosomal rearrangements are of great importance in genetic counseling and prenatal diagnosis in both familial and sporadic cases with idiopathic MR and dysmorphic features. While the deletions of 1pter, 2qter, 4pter and 9pter are seen frequently, the deletion of 9qter is rare. The unbalanced translocations between (10pter;18pter), (18qter;22qter) and (5pter;15qter) were detected for first time in literature.