High-fiber diets have a beneficial impact on glucose metabolism of selected persons with diabetes mellitus. A major concern is the long-term effects of fiber intake on mineral and vitamin status. We measured serum concentrations of selected minerals and vitamins and also assessed three fat-soluble vitamins in 15 patients fed high-fiber diets for an average of 21 mo. Average values for serum calcium, phosphorus, alkaline phosphatase, iron-binding capacity, magnesium, and hemoglobin values were normal. Vitamin B12 and folic acid concentrations in serum were also normal. Indirect assessment suggested that these patients had adequate intakes of the fat-soluble vitamins A, D, and K. These preliminary observations suggest that high-fiber diets containing a wide variety of natural foods are well tolerated for up to 51 mo; we failed to detect evidence suggesting mineral or vitamin deficiency in these patients.