Six patients with multiple myeloma (MM) and a second non-hematological neoplasm (solid tumor) are documented in this study. Two patients had a previous history of adenocarcinoma of the colon prior to MM diagnosis; in three patients a second neoplasm (lung cancer, adenocarcinoma of the urinary bladder and adenocarcinoma of the colon) appeared at the time of MM diagnosis; one patient, a woman with a six-year history of MM, developed hepatoma. The two patients who had had a neoplasm of the colon ten years before and the patient with bladder carcinoma, responded to MM therapy. The patient with lung cancer and the patient with adenocarcinoma of the colon died; the last patient, with MM and liver cancer, is alive but with aggressive disease. In conclusion we have found that in MM patients a second neoplasm may develop or co-exist, in greater frequency than that of the general population.