The advent of non-resupplied space missions of several year's duration poses some unique nutritional problems. Such problems have been of little consequence on missions that have been resupplied with natural foods at intervals of from one to two months or on missions that have been so short that the body's reserves of essential nutrients have not been depleted. Foods used on prolonged, non-resupplied missions must, in addition to providing a nutrient supply sufficient to meet ground-based nutritional requirements, also provide nutrients at levels needed to minimize any adverse effects stemming from weightlessness. Nutritional specifications must also reflect the now well recognized relationships between nutrition and such diseases as cancer and atherosclerosis. The author presents an analysis of crew nutrient needs and shows how these needs might be met on a Mars-bound spacecraft equipped with closed loop systems for providing air and water and carrying expendable food supplies for providing nutrients.