The simultaneous identification, by fluorescence in situ hybridisation (FISH), of each chromosome in a distinct colour became feasible a few years ago. The key question in the application of this and many other developments in molecular cytogenetics to clinical situations is whether the results add significant further information that is relevant to the diagnosis. So far, limited data exist regarding how much improvement the technique brings to the diagnosis of phenotypically abnormal individuals in whom no abnormalities have been detected by conventional G-banding analysis. Because of the lack of a conclusive diagnosis, genetic counselling, estimation of recurrence risk and prenatal diagnosis of these individuals and their relatives is problematic. We report a study with 24-colour whole-chromosome painting of 10 familial and 11 isolated cases with abnormal phenotypes and normal G-banding karyotypes. Previously undetected unbalanced translocations were revealed in two cases. The value and current cost-effectiveness of multicolour FISH for cytogenetic diagnosis is discussed.