Objective: To examine the effect of implementing anxiety disorders guidelines on guideline adherence and patient outcomes in specialized mental health care. Method: A treatment setting in which guidelines were implemented (intervention condition) was compared with one in which guidelines were only disseminated (control condition). Results: Of 61.7% of 81 intervention-condition patients received treatment according to the guidelines vs. 40.6% of 69 control-condition patients (P = 0.01). At 1-year follow-up, intervention-condition patients showed a greater decrease in anxiety symptoms (d = 0.48, P < 0.05); higher percentages of response (52.6% vs. 33.8%; P = 0.025) and remission (33.3% vs.16.9%; P = 0.026); and a greater decrease in the rate of phobic avoidance (d = 0.34, P < 0.05). At 2-year follow-up, control-condition patients had experienced a longer period of treatment, which had eroded most of these differences, except for phobic avoidance. Conclusion: Systematic guideline implementation results in earlier gains and shorter treatment times.