Azathioprine/6MP (AZA/6MP) is effective in long-term treatment (> 3 months) of Crohn's disease and superior to other established medical treatments. The optimal dose remains to be defined. So far, effect has been demonstrated with 2-2.5 mg azathioprine/kg/day, but not with 1 mg/kg/day. A disease controlling effect has been demonstrated during up to four years of continuous treatment, after which data remains to be established. As part of remission-inducing combination therapy the effect of AZA/6MP can not be detected until two-three months after treatment start. High dose intravenous AZA/6MP administration does not shorten this interval. Reversible dose dependent side effects may require dose reduction or termination of treatment. Reversible dose independent side effects exclude further or repeated treatment. Some 10-15% stop treatment due to side effects. There is no increased death rate due to cancer in AZA/6MP treated Crohn patients. When giving the above full dose of AZA/6MP, monthly blood tests are recommended for the entire treatment period, more often during the first three months.