AIMS: To compare the effects on health-related quality of life (HRQoL) of a 2-year intervention with a low-fat diet (LFD) or a low-carbohydrate diet (LCD) based on four group-meetings to achieve compliance. To describe different aspects of taking part in the intervention following the LFD or LCD. METHODS: Prospective, randomized trial of 61 adults with Type 2 diabetes mellitus. The SF-36 questionnaire was used at baseline, 6, 12 and 24 months. Patients on LFD aimed for 55-60 energy percent (E%) and those on LCD for 20 E% from carbohydrates. The patients were interviewed about their experiences of the intervention. RESULTS: Mean body-mass-index was 32.7 ± 5.4 kg/m(2) at baseline. Weight-loss did not differ between groups and was maximal at 6 months, LFD: -3.99 ± 4.1 kg, LCD: -4.31 ± 3.6 kg (p<0.001 within groups). There was an increase in the physical component score of SF-36 from 44.1 (10.0) to 46.7 (10.5) at 12 months in the LCD group (p < 0.009) while no change occurred in the LFD group (p < 0.03 between groups). At 12 months the physical function, bodily pain and general health scores improved within the LCD group (p values 0.042-0.009) while there was no change within the LFD group. CONCLUSIONS: Weight-changes did not differ between the diet groups while improvements in HRQoL only occurred after one year during treatment with LCD. No changes of HRQoL occurred in the LFD group in spite of a similar reduction in body weight.