Monosomy 7 (-7) and deletion of the long arm of chromosome 7, del(7q), are frequent non-random findings in the myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), particularly associated with therapy-related disease (t-MDS and t-AML). The cytogenetic breakpoints of 7q deletions are variable, with both terminal and interstitial deletions reported. It is now believed that most deletions are interstitial, and that the variability in reported breakpoints may be due to the difficulty in determining whether the terminal, pale staining G band is present. It has also been suggested that some reported deletions of 7q may be cryptic translocations. To address these questions, we carried out fluorescence in situ hybridization (FISH) studies on leukaemic cells from a large series of patients using a chromosome 7-specific paint and a 7q telomere-specific probe. Of the 26 cases studied, seven were 'pure' deletions (ie without the involvement of other chromosomes); four were interstitial and two terminal. One further patient had two clones each with a different deletion: one with a terminal del(7)(q22) and the second with an interstitial del(7)(q32-qter). A further nine cases had unbalanced translocations with deletion of 7q terminal sequences. The remaining 10 cases were translocations and complex rearrangements, some involving interstitial deletions of 7q. In two cases in which del(7q) was reported as the sole cytogenetic abnormality by G-banding, FISH revealed cryptic translocations involving 7q.