Sinonasal melanoma is a rare malignancy. We present the clinicopathologic review of 18 cases seen at the British Columbia Cancer Agency between 1976 and 1992: 13 men and five women, mean age 66 years (range 32-88). Patients presented with nasal obstruction and bleeding (n = 8), obstruction alone (n = 4), bleeding alone (n = 5) or pain (n = 1). Those with bleeding presented with a shorter duration of symptoms than those with obstruction alone. All patients with obstruction alone died of their disease, while all patients with bleeding alone are alive or have died of an unrelated cause; four out of eight patients with both obstruction and bleeding are alive. There was no significant relationship between treatment modality and outcome. Histologic subtypes included epithelioid (n = 10), spindle-cell (n = 4), small-cell (n = 3) and pleomorphic (n = 1). Eight out of 11 cases from whom samples of paraffin-embedded tissue were available showed more prominent staining for HMB-45 than for S-100. In two cases, only rare (< 0.1%) cells stained for S-100. Cell type, mitotic rate and P53 expression were unrelated to disease outcome. Six out of seven patients with < or = 10% of cells showing intense staining for PCNA were alive or had died of an unrelated cause, while three out of four with > 10% staining died of their disease.(ABSTRACT TRUNCATED AT 250 WORDS)