OBJECTIVE The objective of this study was to assess age- and sex-related differences of serum carnitine in a Japanese population. METHODS Fasting blood samples were obtained from 296 Japanese males and 258 females 0 to 65 years of age. Serum free carnitine and acylcarnitine levels were determined by a fluorometric method using carnitine dehydrogenase. Of these, serum samples from 20 males 10 to 20 years of age and 23 females 40 to 60 years of age were assayed for testosterone and estradiol, respectively. Fasting blood samples and 24-hour urine samples were also obtained from 20 males and 23 females 0 to 50 years of age for the assessment of renal carnitine reabsorption. RESULTS Serum free carnitine increased with age in children of both sexes, reaching adult value between 15 to 20 years of age in males and between 2 to 10 years of age in females. The mean free carnitine in males (50.3 +/- 7.5 mumol/L) was significantly higher than that in female (41.2 +/- 7.5 mumol/L) between 15-50 years of age (p < 0.05). Serum acylcarnitine remained constant (14.7 +/- 3.3 mumol/L) in each subject of various ages in both sexes. A significant negative correlation was observed between serum free carnitine and estradiol in females (r = -0.55, p < 0.01), but was not observed between serum free carnitine and testosterone in males (r = 0.015). The percent renal reabsorption of free carnitine showed no age-related and sex-related differences. CONCLUSIONS Serum free carnitine level is related to age and sex, while serum acylcarnitine level remains constant. Our findings suggest that estrogen decreases serum free carnitine and causes sex-related differences.