The efficacy of a porcine circovirus type 2 (pcv-2) vaccine was tested in pigs vaccinated at three or six weeks of age. A total of 1106 weaned pigs were randomly allocated to one of three treatment groups: vaccinated at three weeks of age, vaccinated at six weeks of age, or not vaccinated. Each pig was weighed at three, 10 and 22 weeks of age, and 48 pigs selected at random from each treatment group were serially blood sampled at three, six, 10, 14, 18 and 22 weeks of age. The mean weight of the vaccinated pigs was 6·1 kg heavier at 22 weeks than the unvaccinated pigs. The combined mortality and cull rates of the unvaccinated pigs during the growing/finishing period was 14·1 per cent compared with 3·6 per cent and 3·1 per cent for the pigs vaccinated at three weeks and six weeks, respectively. The vaccinated pigs also had a significantly higher mean daily weight gain and a smaller load of humoral pcv-2 than the unvaccinated pigs.