SETTING Authorised clinical mycobacteriology laboratories in Taiwan. OBJECTIVE To evaluate the impact of external quality assessment (EQA) on the quality of drug susceptibility testing (DST) in 2007-2011. DESIGN Panels consisting of 20-30 Mycobacterium tuberculosis strains were used. Efficiency of 95% in detecting resistance to both isoniazid (INH) and rifampicin (RMP), and of 90% to ethambutol (EMB) and streptomycin (SM) was used to define a competent laboratory. RESULTS The proportion of laboratories that fulfilled the competency criteria for all first-line drugs was 16.7% in 2007, increasing to 85.7% in 2008, 86.1% in 2009, 82.4% in 2010, and to 96.8% in 2011 (P < 0.01). The mean efficiency in detecting resistance to INH and RMP reached >99% during 2008-2011 (P = 0.90 for INH and P = 0.82 for RMP), and for EMB it increased from 82.0% in 2007 to 92.2% in 2008 and 99.5% in 2011 (P < 0.01), while that for resistance to SM increased from 82.0% in 2007 to 98.1% in 2008 and 99.5% in 2011 (P < 0.01). Preparations of inoculum for DST and detection of EMB resistance were the main reasons for non-competence. CONCLUSION The EQA programme was effective in improving the competency of clinical laboratories in performing DST for tuberculosis.