Effective therapies for pathological gambling exist, but their use is limited to about 10% of the target population. In an attempt to lower the barriers for help, Internet-based cognitive behavioural therapy (ICBT) has been shown to be effective when delivered to a non-depressed sample with pathological gambling. This study sought to extend this finding to a larger, more representative population, and also test a model to predict responder status. Following advertisement, a total of 284 participants started an 8-week ICBT programme with minimal therapist contact via e-mail and weekly telephone calls of less than 15 min. The average time spent on each participant, including telephone conversations, e-mail, and administration, was 4 h. In addition to a mixed effects model to evaluate the effectiveness of the treatment, two logistic regression analyses were performed with the following eight pre-defined response predictor variables: work-life satisfaction, primary gambling activity, debts due to gambling, social support, personal yearly salary, alcohol consumption, stage of change, and dissociative gambling. ICBT resulted in statistically significant reductions in the scores of pathological gambling, anxiety, and depression as well as an increase in quality of life compared to pre-treatment levels. Follow-ups carried out in the treatment group at 6, 18, and 36 months indicated that treatment effects were sustained. Using the eight predictor variable model rendered an acceptable predictive ability to identify responders both at post-test (AUC = .72, p < .01) and at 36-month follow-up (AUC = .70, p < .01). We conclude that ICBT for pathological gamblers, even if depressed, can be effective and that outcome can partly be predicted by pre-treatment characteristics.