The influence of the clinical condition and the intravenous intake on parameters of fat metabolism has been analyzed. Compared to normal subjects, the correlation between plasma concentrations and the turnover rate of glycerol and free fatty acids varies in the opposite direction in nutritionally depleted and severely injured patients. The significance of plasma concentrations as an index of fat mobilization should be interpreted in relation to the clinical condition. Kinetic measurements are particularly interesting in hypermetabolic patients. Plasma triglyceride and cholesterol concentrations are markedly affected by surgical procedures. Any delay (in treatment) following the injury and the type of intravenous regimen used have an important influence on plasma lipid levels and should be taken into account when groups of patients are studied. The infusion of exogenous fat emulsions significantly affects not only plasma triglyceride levels but also phospholipid and cholesterol concentrations and will modify the plasma lipoprotein pattern. Measurements of plasma clearance and oxidation of fat can be used to evaluate individual tolerance and the metabolic utilization of lipids, but these procedures cannot be easily applied in routine clinical practice. Regular monitoring of plasma triglyceride, cholesterol, and phospholipid concentrations during and after cessation of fat infusion is recommended for each patient who is receiving daily fat infusions so that the safe rate of infusion for that individual can be determined.