The aim of this study was to evaluate the efficacy of an Internet-based weight-loss program for men in an assessor blinded randomized controlled trial. In total, 65 overweight/obese male staff and students at the University of Newcastle (mean (s.d.) age = 35.9 (11.1) years; BMI = 30.6 (2.8)) were randomly assigned to either (i) Internet group (n = 34) or (ii) control group (information only) (n = 31). Both groups received one face-to-face information session and a program booklet. Internet group participants used the study website to self-monitor diet and activity with feedback provided based on participants' online entries on seven occasions over 3 months. Participants were assessed at baseline, 3-, and 6-month follow-up for weight, waist circumference, BMI, blood pressure, resting heart rate, objectively measured physical activity, and self-reported total daily kilojoules. Intention-to-treat analysis revealed significant weight loss of 5.3 kg (95% confidence interval (CI): -7.3, -3.3) at 6 months for the Internet group and 3.5 kg (95% CI: -5.5, -1.4) for the control group. A significant time effect was found for all outcomes but no between-group differences. Per-protocol analysis revealed a significant group-by-time interaction (P < 0.001), with compliers losing more weight at 6 months (-9.1 kg; 95% CI -11.8, -6.5) than noncompliers (-2.7 kg; 95% CI -5.3, -0.01) and the control group (-4.2 kg; 95% CI -6.2, -2.2). Simple weight-loss interventions can be effective in achieving statistically and clinically significant weight loss in men. The Internet is a feasible and effective medium for weight loss in men but strategies need to be explored to improve engagement in online programs.