Micronutrient deficiency not only causes symptoms of severe deficiency, but may also cause more subtle effects on tissue function, including immune deficiency and oxidative damage. The duration of a deficiency state, which is necessary before such effects are clinically significant, is not known. Most biochemical tests are relatively insensitive in detecting changes in micronutrient status, although they do provide a crude index. Many tests are nonspecific, being affected by the acute phase response as well as by nutritional status. Cellular tests are more sensitive and specific than tests in plasma. When interpreted carefully in association with the knowledge of the patient's clinical condition and nutritional intake, laboratory tests can be helpful in diagnosing deficiency states or conditions of excess provision, and in monitoring progress. Well conducted clinical trials of micronutrients in nutritional support are beginning to appear in the literature. Further studies are urgently required that relate outcome to levels of provision and biochemical indices of nutrient status.