An attempt was made to detect the diurnal variation of aqueous protein concentration (aqueous flare) in normal human eyes. The laser flare-cell meter, which was recently developed to determine aqueous protein concentration quantitatively in vivo, was employed in the study. A preliminary study was performed to evaluate the reproducibility of aqueous flare measurements with the laser flare-cell meter in normal subjects, giving a reproducibility coefficient of 12.5%. The aqueous flare intensity was found to be high in the morning and low in the evening. The quotient between the highest and the lowest value was 1.32. The findings were considered to represent changes in actual aqueous protein concentration and the possible sources of this fluctuation were discussed.