To evaluate imagery rehearsal therapy for the treatment of chronic nightmares in a sample of adolescent girls. Adolescent girls ranging in age from 13 to 18 years were recruited from the Wyoming Girls School in Sheridan, Wyoming (treatment group, n = 9; control group, n = 10). These girls had previously suffered a high prevalence of unwanted sexual experiences in childhood and adolescence, and thus many suffered from nightmares, sleep complaints, and posttraumatic stress symptoms. Imagery rehearsal therapy was provided in a 1-day (6-h) workshop. Imagery rehearsal consists of three steps, all of which are performed in the waking state: (a) select a nightmare, (b) "change the nightmare any way you wish," and (c) rehearse the images of the new version ("new dream") 5 to 20 min each day. Control participants received no intervention. At baseline, these girls had been suffering from nightmares, on average, for 4.5 years, and they reported experiencing 20 nightmares per month, which occurred at a frequency of at least one bad dream every other night. At 3 months, self-reported, retrospectively assessed nightmare frequency measured in nights per month decreased 57% (p =.01, d = 1.4) and measured in nightmares per month decreased 71% (p =.01, d = 1.7) in the treatment group, compared with no significant changes in the control group. No significant changes were noted for sleep and posttraumatic stress disorder measures in either group. Imagery rehearsal therapy was an effective treatment option for chronic nightmares in this adjudicated adolescent population.