Measurements of nickel in body fluids, excreta, and tissues from humans with occupational, environmental, and iatrogenic exposures to nickel compounds are comprehensively reviewed. Correlations between levels of human exposures to various classes of nickel compounds via inhalation, oral, or parenteral routes and the corresponding concentrations of nickel in biological samples are critically evaluated. The major conclusions include the following points: Measurements of nickel concentrations in body fluids, especially urine and serum, provide meaningful insights into the extent of nickel exposures, provided these data are interpreted with knowledge of the exposure routes, sources, and durations, the chemical identities and physical-chemical properties of the nickel compounds, and relevant clinical and physiological information, such as renal function. Nickel concentrations in body fluids should not, at present, be viewed as indicators of specific health risks, except in persons exposed to nickel carbonyl, for whom urine nickel concentrations provide prognostic guidance on the severity of the poisoning. In persons exposed to soluble nickel compounds (e.g., NiCl2, NiSO4), nickel concentrations in body fluids are generally proportional to exposure levels; absence of increased values usually indicates non-significant exposure; presence of increased values should be a signal to reduce the exposure. In persons exposed to less soluble nickel compounds (e.g., Ni3S2,NiO), increased concentrations of nickel in body fluids are indicative of significant nickel absorption and should be a signal to reduce the exposures to the lowest levels attainable with available technology; absence of increased values does not necessarily indicate freedom from the health risks (e.g., cancers of lung and nasal cavities) associated with exposures to certain relatively insoluble nickel compounds.