The prevalence of 55 well-defined mild errors of morphogenesis (MEMs) was determined in 100 children with acute lymphoblastic leukemia (ALL), their 80 sibs, 91 mothers, and 76 fathers. Seventy-four patients were treated in Pe ´cs (Hungary) and 26 in Tu ¨bingen (Germany). Only white Caucasian index cases were included in the study. Two-hundred children examined for acute infections served as controls. In addition, we analyzed the family history for birth defects and malignancies, associated major malformations, birth weight, birth order, and pretreatment height of the patients. The results of the Pe ´cs and Tu ¨bingen patients were at first evaluated separately but since no differences were found only the cumulative data were analyzed further. A significantly increased prevalence of MEMs was found in the ALL patients and their sibs of both sexes: their MEM/subject ratios were 1.59 and 1.51, respectively, whereas the same parameter was 0.74 in the mothers, 0.67 in the fathers and 0.69 in the controls. The same tendency was observed when familial cases and/or age-dependent MEMs were excluded and when malformation-type and variant-type MEMs were evaluated separately. No association of ALL with specific MEMs or combinations was recorded. Family history, associated major malformations, parity and birth weight of the patients did not differ significantly from the local reference values, whereas the pretreatment height of the male probands proved to be greater than expected. Am.