Aim The 5:2 diet (two non-consecutive days of 2460 KJ (600 calories) and 5 days of ad libitum eating per week) is becoming increasingly popular. This pilot study aimed to determine whether the 5:2 diet can achieve ≥5% weight loss and greater improvements in weight and biochemical markers than a standard energy-restricted diet (SERD) in obese male war veterans. Methods A total of 24 participants were randomised to consume either the 5:2 diet or a SERD (2050 KJ (500 calorie) reduction per day) for 6 months. Weight, waist circumference (WC), fasting blood glucose, blood lipids, blood pressure and dietary intake were measured at baseline, 3 and 6 months by a blinded investigator. Results After 6 months, participants in both groups significantly reduced body weight (P =