B. mellitensis Rev-1 vaccination is considered the most successful and cost-effective method for prevention and control of brucellosis in small ruminants. The region of Trás-os-Montes e Alto Douro (Northeast Portugal) presented the highest prevalence of brucellosis of the small ruminant population of Portugal. A B. mellitensis Rev-1 mass-vaccination program was carried out from 2001 to 2004 and the effectiveness of this program was evaluated. Brucellosis prevalence in flocks decreased over 20% and positive sheep and goats were reduced by 43%. The prevalence of brucellosis in large flocks was ten times higher than in small ones, higher for sheep than in goats and higher in dairy than in meat flocks. According to flock size, the prevalence of brucellosis may be associated with the absence of biosecurity measures. The high prevalence in sheep was compatible to their predominance because they constitute, on average, 75% of the total of small ruminant population each year. Moreover, the prevalence of brucellosis was higher in flocks with both sheep and goat and intended for dairy production. The results of this work can be used to improve the efficiency of the national eradication program for brucellosis according to the production management of sheep and goats in the vaccinated areas.