Aims To quantify the overall effectiveness of computer-delivered interventions for alcohol and tobacco use. Methods Meta-analysis of 42 effect sizes from randomized controlled trials, based on the responses of 10 632 individuals. Results The weighted average effect size (d) was 0.20, P < 0.001. While lower effect sizes were associated with studies addressing tobacco use (d = 0.14) this may well reflect differences in the types of outcome measure used. Effect sizes did not vary significantly as a function of treatment location, inclusion of entertaining elements, provision of normative feedback, availability of a discussion feature, number of treatment sessions, emphasis on relapse prevention, level of therapist involvement or follow-up period. Conclusion Findings of the meta-analysis suggest that minimal contact computer-delivered treatments that can be accessed via the internet may represent a cost-effective means of treating uncomplicated substance use and related problems.