The Autonomous Community of La Rioja is a region in the north of Spain where the mean annual number of surgical cases of cystic echinococcosis (CE) was 50 (19 per 100000 inhabitants) during the years 1984-1987. This high clinical incidence prompted local authorities to implement a control program in 1986, whose methods and results are reported here. Initially, the program consisted in documenting the prevalence of CE in sheep and humans, communicating an awareness of the disease risks among the population, and treating all registered dogs with praziquantel at intervals of 45 days. Stray dogs were collected systematically, euthanized, and their intestines were examined for Echinococcus granulosus infection. Epidemiological data collected during the course of the program demonstrated that the major reservoirs of E. granulosus were the stray dogs, precisely the ones not receiving periodic praziquantel treatment. Therefore, the program emphasis was shifted to targeting critical points in the transmission of E. granulosus, including improved control of stray dogs, echinococcidal treatments of working sheep dogs, providing means for safe disposal of slaughtered sheep offal and safe disposal of dead sheep in sanitary pits. These measures led to a decline in prevalence of E. granulosus in dogs from 7.0% at the beginning of the program to 0.2% in 2000, i.e. a reduction of 97.2%. Prevalence of infection in adult sheep declined from 82.3 to 20.3%, i.e. a reduction of 75.4%, while the mean number of cysts per infected animal decreased from 6.5% to 0.58 (91% reduction). The rate of diagnoses of new cases in humans between these two dates dropped by 78.9%, from 19 to 4 per 100000 population. In terms of economic costs, these reductions were estimated to yield an increasing cumulative cost/benefit balance that was already positive on year 8 of the program (1994), and that reached 1.96 in year 2000.