Interstitial deletions of the long arm of chromosome 5 (5q-) is the most common structural rearrangement in acute nonlymphocytic leukemia and myelodysplastic syndromes. Owing to the variability of both breakpoints a large number of different deletions occur. However, as banding problems have hitherto precluded the precise localization of the breakpoints, it is still uncertain whether or not different deletions are associated with clinical differences. In this paper relationships between cytogenetic and clinical data were analyzed in 96 5q- cases investigated with high resolution techniques, which allow more precise breakpoint localizations. Twenty-nine cases have not been published before; the others were extracted from the literature. The analyses show: (1) With higher age at diagnosis the proximal breakpoint approaches the centromere. (2) The female patients are older, develop additional chromosome aberrations less often, and tend to live longer than the male patients. (3) del(5)(qI3q33), which is more frequent than any other deletion, tends to be associated with female sex and older age, and shows lower frequencies of additional chromosome abnormalities, and longer survival than the other deletions. It seems to constitute a more benign subgroup than the other deletions. (4) 5q31 is missing in 91 of 93 informative cases and is the most common deleted segment. (5) These results suggest that the deletion of 5q31 may be central in the pathogenesis of 5q- related disorders and that the deletion of other bands has important consequences for prognosis.