Objective: Research suggests that family treatment for adolescents with anorexia nervosa may be effective. This study was designed to determine the optimal length of such family therapy. Method: Eighty-six adolescents (12-18 years of age) diagnosed with anorexia nervosa were allocated at random to either a short-term (10 sessions over 6 months) or long-term treatment (20 sessions over 12 months) and evaluated at the end of 1 year using the Eating Disorder Examination (EDE) between 1999 and 2002. Results: Although adequately powered to detect differences between treatment groups, an intent-to-treat analysis found no significant differences between the short-term and long-term treatment groups. Although a nonsignificant finding does not prove the null hypothesis, in no instance does the confidence interval on the effect size on the difference between the groups approach a moderate .5 level. However, post hoc analyses suggest that subjects with severe eating-related obsessive-compulsive features or who come from nonintact families respond better to long-term treatment. Conclusions: A short-term course of family therapy appears to be as effective as a long-term course for adolescents with short-duration anorexia nervosa. However, there is a suggestion that those with more severe eating-related obsessive-compulsive thinking and nonintact families benefit from longer treatment. J. Am. Acad. Child Adolesc. Psychiatry, 2005;44(7):632-639. Key Words: anorexia nervosa, family therapy.