Ultraviolet-B (UVB) radiation due sunlight can result in sunburns and/or suntans. Sunburn occurs only several hours after solar UVB radiation, while a suntan requires several days to several weeks to develop. In the present study, we measured serum and urine levels of melanin-related metabolites, 5-S-cysteinyldopa (5-S-CD) and 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MI2C), in nine subjects exposed to normal sunlight over the course of 12 months. We collected samples in the middle of each month and examined the variation of the markers, the correlation between them, and their correlation with solar UVB radiation. Those markers exhibited a seasonal variation with lower values in the winter and higher values in the summer. Levels of 5-S-CD and 6H5MI2C in the serum showed 48% and 54% increases in the summer compared with those in the winter, respectively. Comparison of 5-S-CD in the serum and urine showed the highest correlation (r2 = 0.344), followed by the pair of 5-S-CD and 6H5MI2C in the serum. Levels of 5-S-CD in the serum showed the highest correlation (r2 = 0.729) with the mean solar UVB radiation during the first 10 d of the month, while 6H5MI2C in the serum was highly correlated (r2 = 0.483) with solar UVB radiation during the previous month. Levels of 5-S-CD and 6H5MI2C in the serum appear to reflect the degrees of skin injury and pigmentation in the skin, respectively.