We report on 11 patients from 8 independent families (3 pairs of sibs) with a complex clinical pattern including microcephaly, peculiar "bird-like" face, growth retardation, and, in some cases, mild-to-moderate mental deficiency. Most of the patients have recurring respiratory tract infections. One girl has developed B-cell lymphoma. A detailed anthropometric study of 15 physical parameters, including 3 cephalic traits, was performed. It was possible to study the chromosomes of PHA-stimulated lymphocytes in all of the patients. We found structural aberrations with multiple rearrangements, preferentially involving chromosomes 7 and 14 in a proportion of metaphases in all individuals. Profound humoral and cellular immune defects were observed. Serum AFP levels were within normal range. Radioresistant DNA synthesis was strongly increased in all 8 patients who were hitherto studied in this respect. Our patients fulfill the criteria of the Nijmegen breakage syndrome, which belongs to the growing category of ataxia telangiectasia-related genetic disorders. In light of the increased predisposition to malignancy in this syndrome, an accurate diagnosis is important for the patient.