An Internet system was developed for assessing psychopathology, for on-line, protocol-driven cognitive-behavioural psychological treatment and for measuring the effects of treatment. The system focused on the treatment of post-traumatic stress. The treatment comprised 10 writing sessions (45 min each) over five weeks. Participants were assessed on-line before treatment, after treatment and after six-week follow-up. After treatment the participants had improved significantly in terms of post-traumatic stress symptoms (P <0.005) and general psychological functioning (P <0.005), and this was sustained during the follow-up. Nineteen of the 20 participants were clinically recovered after treatment. Reduction in post-traumatic stress symptoms compared favourably to changes in control and experimental groups in trials of similar but face-to-face treatment.