Twenty-five years have elapsed since the first case of del(5q) was discovered in this laboratory. It was reported in 1974 with other subsequently found cases. As the sole anomaly or together with other chromosome changes, deletions of the long arm of 5 have been found in a large variety of hematologic disorders, but especially in myelodysplastic syndromes (MDS) and in acute myelogenous leukemia (AML). Deletions of 5q are the most frequent chromosome anomaly in therapy-induced myelodysplasia/AML. A clinically distinct entity is the 5q- syndrome in which 5q- is the only change in which female sex prevails. Leukemic transformation is low and survival relatively long. Although the myeloid lineage is predominantly affected in 5q- associated disorders, lymphoid disorders, including 5q- acute lymphoblastic leukemia (B and T) exist and show male predominance. The underlying molecular lesions in 5q- myeloid disorders are still largely unknown. There seems to be a critical segment in 5q31, and preliminary studies suggest the inactivation of one or more genes within that region, deletions of which at the molecular level may be heterogeneous.