Paragangliomas of the nasal cavity and paranasal sinuses are extremely rare. This study was conducted to investigate the biologic characteristics of these lesions on different molecular levels and to estimate their biologic behavior. Operative specimens of three patients who underwent surgery for a paraganglioma of the nasal cavity (one case) or paranasal sinuses (two cases) were investigated by routine histology, quantitative DNA analysis, and immunohistochemical assessment of proliferation markers (i.e., Proliferating Cell Nuclear Antigen, PCNA; Ki67-MIB-1), the expression of cell-surface antigens, which reflect the tumor-stroma interaction (i.e., CD 44 v0.4/5 and 6, CD 54, CD 106), oncogene products (nm-23; p53), and bcl-2 as a marker of apoptosis. Histologically, two tumors were paragangliomas of the adenomatous subtype, one lesion was classified as angiomatous. According to DNA analysis, aneuploid cells were detected in all tumors. Two of three paragangliomas were classified as a DNA type III pattern, implying a high percentage of aneuploid cells and an aggressive behavior. Immunohistochemically, paragangliomas of the nasal cavity showed increased scores for both proliferation markers tested, indicating a rapid growth pattern. According to the expression of cell-surface markers and oncogene products, these tumors displayed an aggressive behavior and an infiltrating growth pattern. The highest value for the parameters of quantitative DNA analysis and highest proliferation scores were found in a tumor of a patient who developed multiple tumor recurrences after radical excision of the lesion with clear margins, and finally died of disease. In conclusion, paragangliomas of the nasal cavity and paranasal sinuses examined in this study should be regarded as suspicious concerning their biologic and clinical behavior. Radical excision, and in cases with highly aggressive biology, postoperative radiation therapy are recommended. Tumor biologic examinations can help to recognize high-risk patients for developing recurrences and possibly lesions with a malignant behavior and to enhance our understanding of the biology of these extremely rare tumors of the nasal cavity and paranasal sinuses.