We report three patients in whom non-Hodgkin's lymphomas (NHL) were unexpectedly found during treatment of previously-diagnosed carcinoma. The first patient was a 72 year old woman with an endometrioid carcinoma in whom NHL was found in the pelvic lymph nodes. The second patient was a 76 year-old man with adenocarcinoma of the stomach and NHL of the small intestine. The third patient was a 66 year old man with adenocarcinoma of the prostate and NHL of the iliac lymph nodes. In these patients, it was necessary to exclude metastasis of the known carcinoma and to establish the diagnosis of unexpected NHL. The adjuvant treatment plans of the patients were ultimately not significantly altered, but increased surveillance for recurrence of two neoplasms is now warranted. These patients altered us to beware of indolent NHL in specimens removed for the treatment of carcinoma.