Recent studies with very small numbers of patients showed that in some cases of childhood acute lymphoblastic leukemia (ALL), preleukemic cells are detectable on Guthrie cards that were used for newborn screening. We present here the largest series of ALL patients (n=32) in whom Guthrie cards were analyzed for the presence of preleukemic cells. Rearranged immunoglobulin heavy-chain genes were used as a marker for leukemic clones. We combined our set of patients with 17 previously published cases. Preleukemic cells were detected in 31 of all 49 patients (63%). Positive screening cards were not associated with patient's age at diagnosis but were almost always found in patients with hyperdiploidy (10/11; 91%; P=0.04). High birth weight is an established risk factor for childhood ALL. Positive screening cards were strongly associated with low birth weight (P=0.01). In conclusion, the majority of childhood B-precursor ALL arise prior to birth. In the search for causes of childhood leukemia we should concentrate on prenatal factors as well as postnatal factors. Our results suggest that autologous cord bloods could be a poor choice as the source of stem cells for transplantation in leukemia, which may contain preleukemic cells. Pending the development of suitable methods, childhood leukemia is a potentially screenable disease.