To estimate the risk of venous thrombosis associated with pancreatic malignancies we followed a cohort of patients with pancreatic cancer (n = 202). We calculated incidence rates of venous thrombosis and compared this with population rates using a Standardised Morbidity Ratio (SMR). The effects of location, histology and treatment were assessed by Cox-modelling. The incidence of venous thrombosis was 108.3/1000 patient-years (95% confidence interval (CI) 64.4-163.8), 58.6-fold increased (SMR 58.6, 95% CI 36.9-92.9). Patients with a tumour of the corpus/cauda had a 2-fold increased risk compared with those with a tumour of the caput. Patients treated with chemotherapy had a 4.8-fold increased risk (HR(adj) 4.8, 95% CI 1.1-20.8), whereas radiotherapy did not increase the risk. In a postoperative period of 30 d, patients had a 4.5-fold increased risk of venous thrombosis (HR(adj) 4.5, 95% CI 0.5-40.9). The risk was 1.9-fold increased in the presence of distant metastases (HR(adj) 1.9, 95% CI 0.7-5.1). Anti-thrombotic prophylaxis seems warranted in the first month after surgery, during and after treatment with chemotherapy, and when distant metastases have been diagnosed.