For women attempting pregnancy, obesity reduces fertility and is an independent risk factor for obstetric and neonatal complications. The aim of this evaluator-blinded, randomized controlled trial was to evaluate a weight loss intervention on pregnancy rates in obese women undertaking fertility treatment. Forty-nine obese women, aged ≤37 years, presenting for fertility treatment were randomized to either a 12-week intervention (n = 27) consisting of a very-low-energy diet for the initial 6 weeks followed by a hypocaloric diet, combined with a weekly group multidisciplinary programme; or a control group (n = 22) who received recommendations for weight loss and the same printed material as the intervention. Anthropometric and reproductive parameters were measured at baseline and at 12 weeks. The 22 women who completed the intervention had greater anthropometric changes (–6.6 ± 4.6 kg and −8.7 ± 5.6 cm vs. −1.6 ± 3.6 kg and −0.6 ± 6.3 cm) compared with the control group (n = 17; P < 0.001). The intervention group achieved a pregnancy rate of 48% compared with 14% (P = 0.007), took a mean two fertility treatment cycles to achieve each pregnancy compared with four in the control group (P = 0.002), and had a marked increase in the number of live births (44% vs. 14%; P = 0.02). A group weight loss programme, incorporating dietary, exercise and behavioural components, is associated with a significant improvement in pregnancy rates and live births in a group of obese women undergoing fertility treatment.