Six normal volunteers consumed high protein, high carbohydrate, and high fat isocaloric diets on a metabolic ward for 10 days each. On the high protein diet, serum creatinine, BUN, albumin, and total protein exceeded the normal range in from one to four subjects; the increases were directly related to the percentage of calories derived from protein (R = 0.82, 0.62, and 0.73 respectively; all p less than 0.01). Fasting serum glucose was higher on the high protein diet than on the high fat diet (p less than 0.01); values on the high carbohydrate diet were intermediate. Fasting serum glucose was not significantly related to the percentage of either total carbohydrates or simple carbohydrates in the diet. Within the range of diets frequently ingested in the United States, laboratory values can vary significantly and can exceed the normal ranges in healthy individuals. These variations may mislead clinicians and lead to further unnecessary tests.