Wilms tumor is believed to occur as the result of two mutations affecting both alleles of a critical gene located within the p13 band of chromosome 11 (Knudson and Strong 1972; Riccardi et al. 1978). Several mechanisms by which these mutations occur have already been determined in retinoblastoma (Cavenee et al. 1983) and Wilms tumor (Koufos et al. 1984; Orkin et al. 1984; Reeve et al. 1984; Fearon et al. 1984a; Eccles et al. 1984). Of the various mechanisms, however, no example of a mitotic recombination was demonstrated in Wilms tumor. An example is presented here which has been detected by the use of restriction fragment length polymorphisms (RFLPs) mapping to chromosome 11p. In addition the data presented are consistent with the mapping location of parathyroid hormone (PTH) being proximal to 11p13.