Interphase fluorescence in situ hybridization (FISH) studies with D13S319 show that deletions of 13q14 are common in B-cell chronic lymphocytic leukemia (B-CLL). In contrast, conventional cytogenetic studies in B-CLL seldom reveal abnormalities of chromosome 13. We hypothesized that chromosome 13 anomalies might not be detected because they are caused by cryptic deletions rather than by the absence of dividing B-CLL cells. To investigate this possibility, we used FISH with D13S319 to study metaphases from 12 patients known to have 13q- by interphase FISH. These same patients had normal chromosomes by conventional cytogenetic studies. As a result of this study, we report evidence that B-CLL metaphases with 13q- are not detected because these deletions are often cryptic and not visible by standard G-banding.