Stress has been associated with a variety of chronic and acute conditions and with higher use of health care services. This research reports on 18-month outcomes of a randomized clinical trial of a stress-management program based on the transtheoretical model (TTM; J. O. Prochaska & C. C. DiClemente, 1986). A national sample of 1,085 individuals participated (age range = 18-91 years, M = 55.33; 68.9% female, 31.1% male; 84.8% Caucasian; 15.2% non-Caucasian). Both the treatment and control groups received assessments at 0, 6, 12, and 18 months. In addition to the assessments, the treatment group received 3 individualized reports (0, 3, 6 months) and a manual. The 18-month assessment was completed by 778 individuals (72%). A random effects model indicated that participants completing the study in the treatment group had significantly more individuals reporting effective stress management at follow-up time points than did completers in the control group. Results also indicate that the intervention had significant effects on stress, depression, and specific stress-management behaviors. Results provide evidence for the effectiveness of this TTM population-based stress-management intervention.