The main treatment for pyridoxine nonresponsive cystathionine-β-synthase deficiency is a strict diet. Most centers prescribe low-protein diets based on gram-protein exchanges, and all protein sources are weighed. The purpose of this study is to investigate the effects of a more liberal methionine (Met)-based diet with relaxed consumption of fruits and vegetables on metabolic outcomes and dietary adherence. Ten patients previously on a low-protein diet based on a gram-protein exchange list were enrolled. The natural protein exchange lists were switched to a "Met portion exchange list". Foods containing less than 0.005 g methionine per 100 g of the food were accepted as exchange-free foods. A questionnaire was conducted to assess the effects of the new dietary approach on dietary compliance. The switch to Met portioning had no adverse effects on the control of plasma homocysteine levels in terms of metabolic outcomes. It resulted in a significant reduction in patients' daily betaine dose. The questionnaire results indicated significantly increased dietary compliance, and all patients preferred to continue with this modality. In conclusion, methionine portion-based medical nutrition therapy with relaxed consumption of fruits and vegetables seems to be a good and safe option to achieve good metabolic outcomes and high treatment adherence.