A total of 187 consecutive patients with essential thrombocythemia (ET) were diagnosed and followed by our Hematology Department in the period October 1980-November 1994. The overall follow-up was 773 patient-years. Thrombosis-free survival and overall survival were calculated for the whole cohort; the same parameters were then calculated after arbitrary division of the cohort into two groups, according to the median age at diagnosis (55 years). Fifty percent of the patients had at least one thrombotic episode within 9 years after diagnosis. The thrombosis-free survival curves calculated for patients younger or older than 55 years at diagnosis were comparable. About 85% of the patients were alive 10 years after diagnosis. The survival curves for patients younger and older than 55 years at diagnosis were not significantly different in the observation period, and the observed mortality (seven patients) among patients younger than 55 years at diagnosis was significantly higher than expected (1.68 cases). The relative risk of death was four times greater (SMR = 4.17, 95% C.I. 1.6-8.6, p<0.01) than for healthy, age-matched people living in the same area. Age at diagnosis, smoking, sex, hypercholesterolemia, peak number of platelets, hypertension, and diabetes were not significant prognostic cardiovascular risk factors in our cohort. In conclusion, our data show that ET has to be considered a serious disease that significantly decreases both quality of life (expected life without thrombosis) and life expectancy for younger patients.