Tuberculosis resistant to chemotherapy with two or more antitubercular agents, including at least isoniazid and RIFAMPICIN. The problem of resistance is particularly troublesome in tuberculous opportunistic infections associated with hiv infections. It requires the use of second line drugs which are more toxic than the first line regimens. TB with isolates that have developed further resistance to at least three of the six classes of second line drugs is defined as extensively drug-resistant tuberculosis.