Cellular features of acute lymphocytic leukemia (ALL) in children suggest that it originates in abnormal embryogenesis. Because palmar flexion creases develop in the embryo at the same time as the blood-forming cells, and because both arise from mesodermal tissue, insults to the embryo that may lead to leukemic changes in the blood-forming cells may also result in aberrant palmar crease patterns. This study investigated the relationship between aberrant palmar creases and ALL in children who developed leukemia at age 6 years or younger. Odds ratios and chi squares demonstrated significant differences in bilateral aberrant palmar creases between ALL children and relatives (P less than .025). Differences were not explained by familial clustering of aberrant creases. These results support the theory that the insult occurred during pregnancy, probably in the first trimester. There were no significant differences in either bilateral or unilateral aberrant palmar creases between ALL children and their siblings. All children with bilateral aberrant creases had a higher incidence of central nervous system involvement (50%) than those without bilateral aberrant creases (6%). This may reflect a preleukemic change in utero before the time the blood-brain barrier has been established.