Deletion in chromosome 9p involving the CDKN2A locus (9p21.3) is known in many malignancies. To detect this deletion in adolescent ALL patients we used oligo array CGH and studied 54 patients aged 10-25 years. Deletion rate was 25/54 (46%), of these 19/25 (76%) were homozygous. Small deletions (<200 kb) were found in 8/25 (32%) and the smallest deletion was <30 kb. The only gene affected in all deletions was CDKN2A. We were unable to demonstrate prognostic value of the deletion, however patients with deletion belonged more often (P=0.06) to unfavorable biological category. Our results indicate that CDKN2A deletions <200 kb may not be detected by conventional methods.