['To investigate the main cause of drug-resistant tuberculosis.', 'Clinic isolates of Mycobacterium tuberculosis from 1996 to 2004 in Shanghai were analyzed and the proportion of isolates from new cases (the proxy of primary drug-resistance) and retreatment cases (the proxy of acquired drug-resistance) were calculated. Mycobacterial interspersed repetitive units (MIRU) genotyping was performed and analyzed on pairs of isolates of Mycobacterium tuberculosis from 16 recurrent patients with drug-resistant tuberculosis.', 'Of all 8 120 isolates of Mycobacterium tuberculosis from new cases, 707 isolates (8.7%) were drug resistant. Of all 610 isolates from retreatment cases, 150 isolates (24.6%) were drug resistant. Primary drug resistance accounted for 82.5% (707/857) of all drug resistant isolates. Based on the MIRU genotypes, 13 of all 16 recurrent drug-resistant patients were reinfected with new drug-resistant strain of Mycobacterium tuberculosis.'] Is [ Primary drug-resistance the main cause of drug-resistant tuberculosis ]?