In an uncontrolled pilot study of an interactive, Web-based treatment programme for people with alcohol problems, the mean number of units consumed decreased from 33 per week to 23. A two-arm online randomized controlled trial will be conducted to determine the effectiveness of an interactive Web-based intervention for reducing alcohol consumption. Participants will be randomized to intervention (access to an interactive Web-based intervention plus information) or control (access to an information Web-site only) groups, and followed up for six months. The primary outcome measure will be reduction in units of alcohol consumed in the previous seven days; secondary outcome measures will determine effects on alcohol dependence, alcohol-related harm and social disruption and quality of life. Recruitment, consent, randomization, intervention and follow-up will all be delivered on-line.