The polymorphic restriction fragments of the human Ha-ras locus, produced by the variable tandem repetition (VTR) of a short consensus sequence, fall into three classes based on allelic frequencies. Alleles of the "rare" class (individual frequencies less than 0.5%) have been detected only in white blood cell and tumor DNA of cancer patients. This phenomenon is independent of ethnic origin. No significant association of rare alleles with cancer patients has been demonstrated at an independent tandem repeat locus, VTR4.1. The results suggest that the Ha-ras restriction fragment length polymorphism is useful in cancer risk assessment.