Soluble cell adhesion molecules such as sICAM-1 in serum and other biological fluids are suggested as being useful diagnostic parameters for a variety of diseases. Since increased concentrations during diseases are frequently less pronounced compared to other parameters, we tested whether it would be necessary to align the time of blood collection during the course of a clinical trial. In the 9 volunteers of our trial we found a statistically significant effect at the point in time of blood collection and corresponding serum concentrations of sICAM-1 (p < 0.01). The deviation of the concentrations at a certain time from the daily mean in each individual was seen to be as high as 15%. Our data suggest that daytime variations of serum sICAM-1 concentrations should be taken into consideration when longitudinal observations are planned.