Polybrominated diphenyl ethers (PBDEs) are widely used flame retardants; currently, they are identified as ubiquitous environmental contaminants. Several studies indicate that PBDEs might affect male fertility. We present the results of a pilot study on the relationship between human serum PBDEs and sperm quality. The PBDE levels in Japan are comparable to those found in European countries. Strong inverse correlations were observed between the serum concentration of 2,2′,4,4′,5,5′-hexabromodiphenyl ether and sperm concentration (r = -0.841, p = 0.002) and testis size (r = -0.764, p = 0.01). Extensive studies on the relationship between PBDEs and sperm quality are required.Keywords: polybrominated diphenyl ethers; flame retardants; human serum; sperm. * Correspondence to: Kazuhiko Akutsu Phone: +81-6-6972-1321 FAX: +81-6-6972-2393 E-mail: akutu@iph.pref.osaka. jp 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Polybrominated diphenyl ethers (PBDEs) are used as flame retardants in the production of common consumer products such as electronics, furniture, and textiles. PBDEs are currently recognized as environmental pollutants of global concern because their levels in the environment and in humans have increased markedly over the past several decades (Meironyté et al., 1999;Ikonomou et al., 2002;Akutsu et al., 2003). Since PBDEs are somewhat structurally similar to thyroid hormones such as thyroxine (T4), it was speculated that PBDEs might mimic thyroid hormones and disrupt thyroid homeostasis. Several studies indicate that exposure to PBDEs can decrease the circulating levels of T4 in laboratory animals (Fowles et al., 1994;Zhou et al., 2002) and can cause permanent neurological effects similar to those associated with thyroid hormone deficiencies (Eriksson et al., 2001;Viberg et al., 2004). In addition, several PBDEs possess weak estrogenic/antiestrogenic activities (Meerts et al., 2001). The proliferation and differentiation of Sertoli cells and sperm production are regulated by thyroid and sex hormones. Thus, PBDEs might affect male reproductive health by interfering with the thyroid-and sex-hormone functions. Kuriyama et al. (2005) have reported that developmental exposure to a single low dose (60 µg/kg body weight) of 2,2′,4,4′,5-pentabromodiphenyl ether (PeBDE-99) decreased the sperm count in male Wistar rats. However, no previous studies have examined the relationship between human PBDE levels and sperm quality.We participated in an international project examining the sperm quality of fertile males and found that the sperm concentration of Japanese males was lower than that of European males (Iwamoto et al., 2006). The examination of sperm quality and an estimation of the concentration of chemicals in the serum would be required to reveal the correlation between chemical exposure and the sperm quality in Japanese males. The aim of this pilot study was to measure PBDEs...