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HLA antigens of the A and B loci were determined on the lymphocytes of 30 patients with acute lymphocytic leukemia (A.L.L.), as well as all of their mothers and 26 of the fathers. Seven of the 26 parents shared a common haplotype. This incidence of 269 per 1,000 contrasts with an expected incidence of 90.7 per 1,000, calculated from haplotype frequencies in a North American population (X2=7.61, P smaller.than 0.01) and a frequency of one in 27 in parents of patients with renal failure in the local population (X2=3.91, P smaller than 0.05). There was no statistical difference between the latter group and the North American controls (X2=0.39, P greater than 0.10). This suggests that the genetic background of a large proportion of patients with A.L.L. has restricted heterogeneity, presumably leading to the increased expression of leukemia associated recessive genes.