A number of biological factors have been identified which correlate with prognosis in neuroblastoma. Among these are genetic aberrations, including ploidy, deletions of chromosome 1p and N-myc amplification. Conventional methods of detecting these changes, such as tissue culture for karyotyping and Southern blotting, are time-consuming and yield interpretable results in only a small proportion of cases. We have developed interphase fluorescence in situ hybridisation for use on tumour imprints and bone marrow smears, allowing rapid visualisation of the relevant genetic changes. Valuable prognostic information is therefore available in a few days: the results in our cases were later confirmed by conventional methods. In the foreseeable future it will be possible to define distinct prognostic categories on the basis both of this genetic information and other parameters, and separate therapeutic strategies may then be employed for the different patient groups.