DNA flow cytometry was performed on 44 paraffin blocks from 16 specimens of 11 lesions in five patients from two families identified as probably having the dominant gene of the Lynch Type II syndrome. All but one specimen showed diploidy, although two such specimens were metastases and one a recurrence. The one aneuploid gastric biopsy was followed by a resection with diploidy in all 13 blocks from the malignancy and may represent a superficial change without clinical significance. The prolonged survival after discovery of malignancy in such families may be explained in part by diploidy of the lesions.