We have investigated the diagnostic value of fluorescence in situ hybridisation (FISH) to detect t(11;14) and trisomy 12 in 53 cases with a B cell leukaemia difficult to classify on clinical and laboratory grounds. These cases were initially diagnosed by morphology and immunophenotype and in 33 of them, on tissue histology, as follows: chronic lymphocytic leukaemia (CLL), 20, 18 of them with atypical features; B cell prolymphocytic leukaemia (B-PLL), two; mantle-cell lymphoma (MCL), 15; splenic lymphoma with villous lymphocytes (SLVL), five; lymphoplasmacytic lymphoma, six; follicular lymphoma, one and, four cases remained unclassifiable. FISH demonstrated BCL-1 rearrangement in the circulating cells from 15 cases classified as: MCL (10), atypical CLL (three) and B-PLL (two). A definitive diagnosis of MCL was made on review of the spleen histology in one out of the three atypical CLL with BCL-1 rearrangement. Trisomy 12 was detected in eight cases which included four atypical CLL, one typical CLL, two MCL and one unspecified B cell lymphoma by histology and morphology. One of the MCL had both trisomy 12 and BCL-1 rearrangement and the other was CD5+, CD23+ and had a CLL score of 3, suggesting the latter diagnosis. Our findings demonstrate that FISH analysis is useful to clarify the nature of the disease in patients presenting with a B cell leukaemia in which the diagnosis is difficult by conventional methods. FISH established with certainty the diagnosis of MCL by showing BCL-1 rearrangement in over two-thirds of cases in which this was suspected, including blastoid forms, and confirmed the diagnosis of most cases of atypical CLL.