Paraneoplastic syndromes (PNS) are remote effects of cancer that are, by definition, caused neither by invasion of the tumor or its metastases nor by infection, ischemia, metabolic and nutritional deficits, surgery or other forms of tumor treatment. The purpose of the current review was to present the challenging elements of differential diagnosis in oncology, as they may represent the main clinical problem in a patient diagnosed with cancer, even though the complete knowledge of both their clinical aspects and pathogenesis remain quite poor. This review focuses on the paraneoplastic syndromes related to dermatology and rheumatology, as the most frequent manifestations come from connective tissues that might determine a patient to ask for consultation by a general practitioner.