The prevalence, clinical presentations, and diagnostic significance of the paraneoplastic syndrome (PNS) in the setting of a department of internal medicine in a community hospital was studied. During a 7-year period (1979–1985) a total of 167 patients among 11,000 hospitalized patients were diagnosed as having a malignant neoplasia previously unknown. From this group we selected all cases who presented with one or more of the known clinical PNSs. Forty-two cases, i.e., 25% fulfilled the selection criteria. Their prevalence in our population was higher than usually found in the literature. Sixteen different categories of PNS were observed. They occurred 55 times, since more than one PNS was present in 9 cases. No significant correlation was observed between any type of PNS and any particular class of malignant neoplasia. At the time of initial evalution, PNSs were associated with stage I malignancy in 15 cases, with stages II and III in 8 and 3 cases, respectively, and with stage IV in 22 cases. PNSs were the leading symptom or sign in 56% of the patients. In these patients the PNS determined the direction of work-up in the search for a malignant tumor. However, PNSs were essential for suspecting cancer in stage I of the disease in 6 cases only, i.e., 14%. We stress that awareness of the clinical implications of these syndromes may permit an earlier diagnosis of malignanc