Gain of chromosome 7 represents one of the most frequent cytogenetic findings in B-cell lymphomas with a follicular growth pattern. We used fluorescence in situ hybridization (FISH) and a probe specifying chromosome 7 on lymph node imprints and/or bone marrow (BM) and peripheral blood (PB) smears from six consecutive patients with follicle centre lymphomas (FCLs) grade I or II (low-grade lymphomas), four patients with FCLs grade III and 11 patients with diffuse large B-cell lymphomas (DLBCLs) (high-grade lymphomas). We found gains of chromosome 7 in 14/18 successfully analysed cases (i.e. 2/6 FCLs grade I-II, 3/3 FLCs grade III and in 9/9 DLBCLs) using lymph node imprints. Moreover, the FISH technique demonstrated gains of chromosome 7 in 1/4 BM and 0/4 PB samples from FCLs grade I-II, in 2/4 BM and 2/4 PB specimens from FCLs grade III and in 4/9 BM and 2/9 PB samples from the DLBCLs. In contrast, morphologically recognizable lymphoma cells were seen in only 1/4 BM and 0/4 PB samples from the FCLs grade III and in 1/11 BM and 1/11 PB samples from the DLBCLs. We conclude that: (i) gain of chromosome 7 marks the progression from indolent to aggressive FCL and would appear to be a common finding in patients with FCLs grade III and in DLBCLs, (ii) clonal lymphoid cells occur frequently in BM and PB in high-grade lymphomas, making traditional staging by cytomorphology uncertain, and (iii) using gains of chromosome 7 as a marker of lymphoma cells, FISH is a useful method to detect minimal residual disease in FCLs grade III and DLBCLs.