The intra-individual variation in the concentrations of serum cholesterol, triglycerides and high density lipoprotein cholesterol (HDL-chol) was determined in 53 healthy subjects, without extreme standardization of test subjects and sampling conditions. Within 1 year, the intra-individual variation of the subjects ranged from 3.9 to 10.9% for cholesterol; from 12.9 to 40.8% for triglycerides, and from 3.6 to 12.4% for HDL-chol. More than 60% of the average total intra-individual variation was caused by biological fluctuations and the remainder was the result of analytical variation. Thus, a single measurement of these serum constituents in an individual can be misleading or meaningless, unless the value is considerably outside the normal range. No significant diurnal variation was found in the concentrations of serum cholesterol and HDL-chol. The maximal post-prandial increase of the serum triglycerides was twice as great in the men than in the women. Finally, significant trends in the fluctuations of serum lipids and HDL-chol during 1 year were not found.