Aims To test the efficacy of a therapist-guided high-intensity internet intervention compared with an unguided low-intensity internet intervention among individuals with alcohol use disorder. Design A three-group randomized controlled trial with follow-up assessments post-treatment (12 weeks) and 6 months post-randomization (primary end-point). Settings General population sample in Sweden. Participants A total of 166 on-line self-referred adults (49% males) with a score of 14 (females)/16 (males) or more on the Alcohol Use Disorders Identification Test, a preceding week alcohol consumption of 11 (females)/14 (males) or more standard drinks and an alcohol use disorder according to a diagnostic interview. Interventions and comparators Both the high- (n = 72) and low-intensity internet interventions (n = 71) consisted of modules based on relapse prevention. Controls were on a waiting-list (n = 23), and were only followed until the post-treatment follow-up. Participants were randomized at a 7 : 7 : 2 ratio. Measurements Primary outcome was self-reported alcohol consumption in the preceding week measured as (1) number of standard drinks and (2) number of heavy drinking days at the 6-month follow-up. Findings Alcohol use disorders were largely in the severe category (74.7%), with the majority of participants having had alcohol problems for more than 5 years. Attrition was 13 and 22% at the post-treatment and 6-month follow-up, respectively. At the 6-month follow-up, an intent-to-treat analysis showed no significant differences in alcohol consumption between the high- and low-intensity interventions [standard drinks d = -0.17, 95% confidence interval (CI) = -0.50 to 0.16; heavy drinking days: d = -0.07, 95% CI = -0.40 to 0.26]. Prevalence of negative effects was somewhat low (8-14%) in both intervention groups, as was deterioration (3-5%). Conclusions At 6-month follow-up, there were no significant differences between a therapist-guided high-intensity internet intervention and an unguided low-intensity internet intervention in reducing alcohol consumption among individuals with an alcohol use disorder.