Heritability of the unilateral-sporadic (non-familial) form of Wilms tumor was examined in the offspring of 96 long-term survivors of the neoplasm. No Wilms tumor has developed in any of the 179 offspring of these patients. The maximum likelihood estimate of a hereditary Wilms tumor in our patients is zero and the corresponding 95% upper confidence limit ranges between 0.06 and 0.11, depending on penetrance. Among their offspring, the 95% upper bound of the risk of Wilms tumor is 0.02. These figures can be applied in genetic counseling of other survivors of unilateral-sporadic Wilms tumor.