Thirty-one of thirty-three children and adolescents with leukemia had a history of object loss or separation, or threat of such loss. The subsequent depression may be one of the factors determining manifest development of leukemia. The authors studied the relation of mother and child in prenatal months, infancy, and childhood. Prenatal factors include somatic and psychological events which affect the mother at crucial points of foetal hematopoietic morphogenesis. Continuing depressive reactions in the mother affect the child's and mother's capacity for object relationship. To the mother the child is a lost object of her own past. The child's capacity is affected by such happenings as recurrent infections (many children had such a history) and its mother's availability. Depression in the