In 26 consecutive patients operated for Wilms' tumour samples from the tumour were genetically analyzed. Clonal acquired chromosome aberrations were found in 13 patients and a constitutional trisomy 18 as the sole change in 1. The chromosome number was altered in 13 patients. Numerical changes occurred in 16 patients and breakpoint of chromosome 1 in 6 patients. There was no structural alteration of chromosome 11. The observed cytogenetic heterogeneity illustrates the complexity of genetic changes involved in the genesis and progression of Wilms' tumour. To further elucidate the phenotypic impact of chromosomal aberrations the correlation to histology and the clinical course will be important.