Patients with the heritable form of retinoblastoma carry a constitutional mutation in the retinoblastoma locus in heterozygous form. The majority of such cases are the result of new mutations, which may be inherited by their offspring. We have identified such constitutional mutations within the retinoblastoma locus in 3 out of 66 investigated unrelated gene carriers, using Southern blot analysis and Rb-gene cDNA-probes. The identified mutations were found to be located in different regions of the gene. These analyses may be used to identify or exclude close relatives at risk for the disease. In 2 of the 3 cases, the identified aberrations were used for informed genetic counselling of relatives.