Normal tumour-adjacent breast tissue samples from 12 breast cancer patients forming six monozygotic twin pairs were analysed for loss of heterozygosity (LOH) on chromosomes 1, 13 and 17. 7 patients showed LOH at one or more markers. Each of them had a different LOH pattern. Only one twin pair showed LOH at the same locus, but the twins had lost a different allele. Multiple (n=1-13), histologically normal samples were collected from 6 bladder cancer patients and analysed for LOH on chromosomes 3 and 9. On chromosome 9, all 6 patients analysed showed LOH in at least one sample and one marker. Four of them also showed LOH on chromosome 3. Samples surrounding different tumours of a given patient resembled each other. More heterogeneity was seen between the patients, even though they shared some similarities in LOH clustering. The results demonstrate that tumour-adjacent normal tissues already harbour genetic changes typical for tumours. These alterations can reveal the earliest changes leading to tumorigenesis.