We have collected and studied the genealogical data of 8 patients with the autosomal recessive syndrome of microcephaly, normal intelligence, immunodeficiency, risk of malignancy and chromosomal instability resembling ataxia telangiectasia (AT), but different in complementation group. 50% of our probands died from lymphoreticular malignancies in early childhood. We have found a significantly increased incidence of malignant tumors in 142 blood relatives as compared with a control group of 87 spouses. All patients belonged to the same complementation group differing from the 5 known AT complementation groups, which seems to be in general more malignant than all other groups of AT. From this standpoint our material is homogeneous in contrast to other similar studies in AT families. We think this syndrome represents another model to examine the relationship between genetic background, chromosomal abnormalities, immunodeficiency and cancer development.