Estimate of survival of 867 patients with essential thrombocythemia, by IPSET score. Three risk factors were taken into account: age ≥ 60 years (2 points), leukocyte count ≥ 11 × 10⁹/L (1 point) and prior thrombosis (1 point). The resulting risk categories were low (sum of scores = 0), intermediate (sum of scores = 1 or 2), and high (sum of scores = 3 or 4) with significantly different survivals: not reached in low-risk patients, 24.5 years (95% CI: 22.3-NR) in intermediate risk, and 14.7 years (95% CI: 11.9-18) in high-risk patients.