Between 1987 and 1991, cytogenetic studies were carried out on small round cell tumours of 68 patients from the Northern Health Region of England. Clonal chromosome abnormalities were found in 30, comprising 15 neuroblastomas, 7 Ewing's tumours, 7 rhabdomyosarcomas, and 1 granular cell tumour. Characteristic rearrangements were found in five cases of Ewing's tumour [all with translocation t(11;22) (q24;q12)] and in four cases of rhabdomyosarcoma [all with evidence of translocation t(2;13) (q35-37;q14)]. In one case of Ewing's tumour and three of rhabdomyosarcoma, the cytogenetic findings were important in diagnosis. Within the neuroblastomas, examples were found of hyperdiploidy, 1p rearrangements, double minute chromosomes, and homogeneously staining regions, but too few cases were available for prognostic associations to be assessed. Our findings confirm the diagnostic importance of chromosome abnormalities in small round cell tumours and indicate that cytogenetic analysis should be an intrinsic part of the initial investigations of all patients with such tumours.