Patients with polycythaemia vera (PV) or essential thrombocythaemia (ET) have an increased risk of arterial and venous thromboembolic complications. Since hyperhomocysteinaemia (HHC) is a risk factor for vascular disease, we investigated the frequency of HHC in these disorders and analysed a possible association of elevated plasma homocysteine levels with vascular complications. In the cohort of 134 patients from Vienna (69 female, 65 male, median age 65.5 years, range 21-91 years) with PV (n = 74) or ET (n = 60), plasma homocysteine levels were significantly higher compared to 134 healthy controls. Median homocysteine level was 12.3 micromol/l (range 3.5-48.4 micromol/l) in patients with PV or ET and 8.9 micromol/l (range 4.8-30.5 micromol/l) in normal controls (P < 0. 0001). In addition to the 134 patients from Vienna, 48 patients (28 female, 20 male; median age 66.5 years, range 24-82) from Vicenza with PV (n = 25) or ET (n = 23) were included to evaluate the impact of HHC on the risk of thrombosis. Of 59 patients with HHC (44 from Vienna and 15 from Vicenza) 18 (31%) had a history of arterial and 10 (17%) of venous thrombosis. Of 123 patients with normal homocysteine levels, 30 (24%) had arterial and 16 (13%) had venous thromboses. The difference between the two groups was statistically not significant. Even though mild to moderate HHC occurred in a larger number of patients with PV or ET and thrombosis, it can presently not be regarded as an additional risk factor for thrombosis.