We observed adult T-cell leukemia (ATL) associated with cancer in four of 46 patients with ATL, including gastric cancer in two, lung cancer in one, and esophageal and gingival cancer in one. The associated cancer was recognized in three of these patients prior to the diagnosis of ATL and, in one, at autopsy. One patient suffered from a skin type of ATL, two a smoldering type and one a chronic type. No information was obtained from our patients presented here as to whether HTLV-1 influenced the appearance of other cancers, or whether the treatment for cancers influenced the appearance of ATL. Further investigations are necessary to clarify the relationship between ATL and associated cancers.