We determined the primary and secondary resistance of isolates of M. tuberculosis to the standard anti-tuberculous drugs in Karachi (Pakistan). Primary resistance to one or more anti-tuberculous drugs was found in 17% of 123 isolates of M. tuberculosis (obtained from patients with no history of previous treatment for tuberculosis). Secondary resistance was found in 36% of 33 isolates (obtained from individuals who had received anti-tuberculous treatment in the past). The drug to which organisms were most commonly resistant was isoniazid (11% primary resistance, 30% secondary resistance). Fifteen per cent of isolates obtained from previously-studied patients showed secondary resistance to rifampicin. We discuss the importance of these findings for tuberculosis treatment and control.