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Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by telangiectasia, progressive ataxia, sinopulmonary infections and a combined immunodeficiency (usually consisting of IgA deficiency, IgE deficiency, IgG2 and IgG4 deficiency and a disturbed T cell immunity). The alpha-fetoprotein level is elevated. Cytogenetic studies reveal a very specific chromosome instability with multiple chromosome 7 and/or 14 rearrangements (preferential breakpoints 14q32, 14q12, 7q35 and 7p12). X-ray hypersensitivity is one of the hallmarks of the disease. Nijmegen Breakage Syndrome (NBS), an autosomal recessive disorder with some features of AT, was first reported in 1981. At this moment at least 19 patients have been recognized. Clinical symptoms are microcephaly from birth, a peculiar face, growth retardation, repeated respiratory tract infections and renal abnormalities. Immunological, cytogenetic and cell-biological findings in NBS are identical to AT. However, alpha-fetoprotein levels are not increased. A tendency toward malignancy has been demonstrated in both syndromes. Recently, we encountered three patients with variants of these syndromes.