This study investigates the relapse rate among multibacillary leprosy patients treated with 24 months of MDT in south west China. A retrospective relapse survey was conducted in the southwest of China. A detailed questionnaire was designed to collect the data on relapse among MB patients who completed 2 years of the WHO/MB regimen, from 1989 to 2000. The data about 2517 multibacillary leprosy patients in 27 counties in the southwest of China were collected. Among 2517 MB patients, 235 patients died or were lost to follow-up and 2374 were followed up for more than 3 years after completion of MDT. The total duration of follow-up was 20,825 person-years, with a mean duration of 8.27 years per patient. Five patients with relapse were identified with an accumulated relapse rate of 0.21/1000 person-years. Their initial BIs ranged from 1.8 to 5. The patients with relapse occurred 48-158 months after the completion of MDT. The relapse rate of MB patients treated with 24 months of the WHO/MB regimen was observed to be very low after long-term follow-up.