The role of host factors in the etiology of carcinoma of the ureter has received only limited attention. We describe 4 Lynch syndrome II kindreds in which carcinoma of the ureter, as well as other forms of urological cancer, have occurred either in obligate gene carriers and/or in association with multiple primary cancers that are integral to this autosomal dominantly inherited disorder. These new findings are in accord with increasing recognition of tumor heterogeneity in the Lynch syndrome II, which currently, in addition to an excess of proximal colonic cancer as well as carcinoma of the endometrium, ovary and small bowel, also may include carcinoma of the ureter as well as other urological forms of cancer as integral lesions in subsets of these families. These observations have important ramifications for better understanding of the etiology, pathogenesis and control of urological cancer in this disorder.