It is the purpose of this review to analyse prevalence and pathogenesis of tumours occurring in the diverse conditions of primary and secondary immunodeficiencies. In general, most states of immunodeficiences are connected with a highly increased tumour risk. However, there is circumstantial evidence that immunodeficiency-related malignancies do not primarily result from an impaired immunosurveillance. With special regard to ataxia-telangiectasia it has been found that congenital chromosomal instability deregulates the maturation of immunocompetent lymphoid cells by interfering with their natural DNA rearrangements and eventually leads to false recombinatory events with oncogenetic consequences in a parallel way. Malignancies in primary common variable or in secondary (e.g. AIDS) immunodeficiency may result from an impaired immunity to oncogenic viruses or from chronic immune damage due to secondary autoimmune disorders.