In five severely head-injured patients we determined the plasma concentrations of apolipoproteins serum amyloid A, A-I, A-II, C-III, and B, prealbumin and C-reactive protein on day 1, 5, 10 and 15 after head injury where possible. A dramatic increase in apolipoprotein serum amyloid A up to a mean plasma level of 0.764 g/l was accompanied by a considerable decrease in apolipoprotein A-I, apolipoprotein A-II and apolipoprotein C-III concentrations. The variations observed by immunological methods were confirmed by two-dimensional gel electrophoresis performed on plasma and different lipoprotein fractions. In addition to its association with high density lipoproteins, apolipoprotein serum amyloid A was also found with lipoproteins of low and very low density. Two-dimensional electrophoresis also showed the presence of several different serum amyloid A-peptides not seen in plasmas from healthy subjects. We propose that apolipoprotein serum amyloid A may be responsible for the decrease of the main HDL apolipoproteins in head-injured patients.