The allele distribution of the Ha-ras gene on chromosome 11p was analysed by the restriction fragment length polymorphism of the enzymes Mspl/Hpall in 238 individuals. The investigation covered 116 patients with colorectal carcinoma and 122 patients with Crohn's disease, representing two patient populations with the same ethnic origin, one with a malignant and the other a benign disease of the same organ system. A total of 17 different alleles were detected belonging to the common, intermediate, and rare classes according to the original nomenclature of Ha-ras alleles. Patients with Crohn's disease showed no difference in the distribution of Ha-ras alleles when compared with expected frequencies. In patients with colorectal carcinoma, the frequency of rare alleles was significantly increased compared with the patients with Crohn's disease (chi 2 = 8.166; Fisher's exact test = 0.005) and with a reference population of 424 cancer free individuals (chi 2 = 49.312; Fisher's exact test = 0.000). Homozygosity was not detected for any rare allele. The occurrence of a rare Ha-ras allele was not linked to the location of the colorectal tumour. These results confirm the hypothesis that unique Ha-ras alleles represent an inherited factor which predisposes the development of colorectal cancer.