Background: Heart and lung transplant recipients are at a substantially increased risk of cardiovascular disease (CVD). Since both low-fat and Mediterranean diets can reduce CVD in immunocompetent people at high risk, we assessed adherence among thoracic transplant recipients allocated to one or other of these diets for 12 months. Methods: Forty-one transplant recipients (20 heart; 21 lung) randomized to a Mediterranean or a low-fat diet for 12 months received diet-specific education at baseline. Adherence was primarily assessed by questionnaire: 14-point Mediterranean diet (score 0-14) and 9-point low-fat diet (score 0-16) respectively, high scores indicating greater adherence. Median scores at baseline, 6 months, 12 months, and 6-weeks post-intervention were compared by dietary group. We further assessed changes in weight, body mass index (BMI) and serum triglycerides from baseline to 12 months as an additional indicator of adherence. Results: In those randomized to a Mediterranean diet, median scores increased from 4 (range 1-9) at baseline, to 10 (range 6-14) at 6-months and were maintained at 12 months, and also at 6-weeks post-intervention (median 10, range 6-14). Body weight, BMI and serum triglycerides decreased over the 12-month intervention period (mean weight - 1.8 kg, BMI -0.5 kg/m2, triglycerides - 0.17 mmol/L). In the low-fat diet group, median scores were 11 (range 9-14) at baseline; slightly increased to 12 (range 9-16) at 6 months, and maintained at 12 months and 6 weeks post-intervention (median 12, range 8-15). Mean changes in weight, BMI and triglycerides were - 0.2 kg, 0.0 kg/m2 and - 0.44 mmol/L, respectively. Conclusions: Thoracic transplant recipients adhered to Mediterranean and low-fat dietary interventions. The change from baseline eating habits was notable at 6 months; and this change was maintained at 12 months and 6 weeks post-intervention in both Mediterranean diet and low-fat diet groups. Dietary interventions based on comprehensive, well-supported education sessions targeted to both patients and their family members are crucial to success. Such nutritional strategies can help in the management of their substantial CVD risk. Trial registration: The IRAS trial registry (ISRCTN63500150). Date of registration 27 July 2016. Retrospectively registered.