We evaluated the influence of aging, gender and indications of anticoagulant therapy on acenocoumarol requirements in 1845 patients from 30 to 99 years old receiving acenocoumarol therapy who were monitored in our hospital outpatient anticoagulation clinic from March 1993 through September 1999. The patients were stratified in seven age groups, comprising older than 80 years and the five decades between 30 years and 80 years. We found a progressive decrease in the acenocoumarol requirements from 30 years to 80 years (rho=-0.98), which was estimated as 2.7 mg/week per decade (11.5% per decade). This decrease was not the consequence of a different range of anticoagulation or differences in body weight. We did not find correlation between the decrease of acenocoumarol requirements and different biochemical parameters including, creatinine, calcium and alanine aminotransferase. We detected a progressive decrease in levels of serum total proteins but changes in this parameter did not correlate with the amount of acenocoumarol requirements. The dose of acenocoumarol (mg/week per patient) of those patients suffering from venous thromboembolism were higher than the remainder of the patients (18.4 +/- 9.3 versus 14.5 +/- 7.8, P <0.0001). This finding was also detected, after stratifying the patients by decades, from 60 years to 80 years. In conclusion, requirements of acenocoumarol decrease with aging; this decrease represents an important amount from 30 years to 80 years and it should be kept in mind to choose the initial dose of acenocoumarol. Patients with venous thromboembolism required a higher dose of acenocoumarol.