The following is a table of reference values for adults for laboratory tests commonly ordered at the Massachusetts General Hospital (MGH) and recorded in the Case Records. The table revises the most recently published data (Normal Reference Laboratory Values. N Engl J Med 1998; 339: 1063-72). Laboratory values are expressed in the units used at the MGH and the units of the Système International d’Unités (SI units). The table is not intended to provide a comprehensive review of reference values, since this information is widely available in standard textbooks. To avoid suggesting an endorsement of commercial products by the hospital or the Journal, information on specific methods and instruments is not provided. Reference values are affected by many variables, including the patient population and the laboratory methods used. The ranges used at the MGH may therefore not be appropriate for other institutions and may not be optimal in some situations.