We report the response to, and toxicity of antithymocyte globulin (ATG) treatment in 11 consecutive patients (7 men and 4 women; median age 46 years) with aplastic anemia (AA). Six of the patients had severe disease and 5 had moderate disease; all were treated within one year from diagnosis. The ATG regimen was the initial treatment for 6 patients, but a sequential treatment for the other 5. Cyclosporin A was administered orally 1-3 weeks after the ATG treatment. All patients were assessed for over 6 months (median, 20.4 months); 8 showed a good response, 1 a minimal response, and 2 no response. Disease severity had no influence on the response. In 1 patient, ATG treatment had to be discontinued because of hepatic toxicity. However, adverse reactions were not severe in the other 10 patients. These findings suggest that ATG treatment is a safe and effective therapy for AA.