We present a female patient with monoclonal gammopathy of undetermined significance who has remained stable for five years but evolved to overt myeloma in strict temporal relationship with the diagnosis of GH-secreting pituitary macroadenoma. IGF-I serum levels correlated with serum and urine M component. Since the in vitro role of IGF-I on proliferation and survival of normal and neoplastic plasma cells has been recently emphasized, the pathogenetic link between acromegaly and transformation of gammopathy to overt myeloma in this case is discussed.