Despite the efficacy of cognitive-behavioral therapy (CBT) in treating adolescent anxiety, few sufferers seek treatment. Barriers to accessing psychologists include a shortage of skilled therapists, long waiting lists, and affordability. The Internet is a medium possibly able to address issues of accessibility and affordability. This study aimed to assess the efficacy of online therapy in the treatment and prevention of adolescent anxiety and depression. Participants (N1/438) were randomly allocated to one of four conditions: online CBT, face-to-face CBT, combined face-to-face=online CBT, and control. Combined face-to-face=online CBT is more effective in treating symptoms of depression and anxiety compared to stand-alone online or face-to-face therapy. The present study suggests that for those who are unable to access face-to-face therapy, computerized therapy may be a viable option. This is an important finding, especially in light of current capacity to treat and accessibility problems faced in the treatment of adolescent depression and anxiety.