Between 1988 and 1992 ultrasound-guided fine needle aspiration biopsies of thromboses in the main branches of the portal vein were carried out in 15 patients with liver cirrhosis. The aims of the study were to evaluate the usefulness, feasibility and diagnostic accuracy of this procedure in cirrhotics with known or suspected hepatocellular carcinoma. The procedure was carried out only in patients with a platelet count ≥40 000/μL and prothrombin activity ≥40%. A single pass, with a 22 gauge spinal needle, was performed in the portal vein lumen. Diagnosis of the aetiology of the portal vein thrombosis was obtained in all 15 cases. In 12 cases, a cytological diagnosis of hepatocellular carcinoma was made. In one case, the neoplastic cells aspirated were compatible with adenocarcinoma, and a subsequent colonoscopy confirmed the presence of colonic cancer. The material aspirated was compatible with chemically-induced thrombosis in one patient who had undergone several percutaneous ethanol injection sessions for treatment of hepatocellular carcinoma, and in the last case only blood was aspirated, thus ruling out the coexistence of hepatic cancer. We conclude that fine needle aspiration biopsy of portal vein thrombosis is a feasible, low risk procedure that facilitates the diagnosis of hepatocellular carcinoma when fine needle biopsy of focal liver lesions fails. Fine needle aspiration biopsy of portal vein thrombosis is also useful in excluding neoplastic aetiology of portal vein thrombosis.