The effects of spinach in the diet on mineral balances were examined in 12 men who consumed three controlled diets for 4 wk each. Diet 1, a low-fiber diet, contained spinach, which is high in oxalic acid, every other day. Diet 2 contained fiber in fruits and vegetables, including spinach every other day. Diet 3 was the same as diet 2 except that cauliflower, which is low in oxalic acid, replaced the spinach. During wk 4, mean balances of calcium, magnesium, and zinc on diet 2 were negative and significantly lower than those on diet 1. Magnesium and zinc balances during wk 4 were also significantly lower on diet 2 than on diet 3. Mean balances of these minerals were not significantly lower on diet 2 during wk 3, nor were there any significant differences among mineral balances when wk 3 and 4 were considered together. Fecal oxalic acid was significantly greater on diet 2 than on diet 1 during both wk 3 and 4.