Wilms tumor was discovered in a 22-month-old otherwise healthy girl. Her pretreatment karyotype proved to be normal 46,XX with total premature division (PCD) in 21% of her lymphocyte mitoses. This phenomenon was not seen in the parents. PCD was found in less than 4% of lymphocyte mitoses of five other children with Wilms tumor. The individual finding provides further evidence for a relation of PCD to tumorigenesis; however, elucidation of the question needs further systematic studies.