There are a number of potential causes of failure in the treatment of pulmonary TB, but some are unimportant in practice. Criteria of failure are suggested. Default from treatment is the commonest cause of treatment failure. Various remedies are discussed. Common errors are outlined. In many countries, a major educational effort is needed to ensure that all doctors treating TB are aware of potential causes of failure and how they can be avoided. The only drug regimen which should be used are those which have been proved by large scale, controlled trials to give virtually uniform success. Knowledge of these regimens needs to be regularly updated.