Zeranol (3,4,5,6,7,8,9,10,11,12-Decahydro-7,14,16-trihydroxy-3-methyl-1H-2 - benzoxacyclotetradecin-1-one) is a synthetic oestrogenic agent used as an animal growth promoter. The effects of occupational exposure to zeranol in 11 exposed workers from a pelletising plant and 14 nonexposed subjects were assessed. A questionnaire showed that more breast symptoms were reported by male and female plant workers compared with non-exposed subjects, although the difference was not statistically significant. Clinical assessment showed no cases of gynaecomastia in all the male participants. Blood samples analysed by high performance liquid chromatography for zeranol, its precursor zearalenone, and its main metabolites did not show any of these compounds above the laboratory limit of detection. Serum levels of follicle stimulating hormone (FSH), luteinising hormone (LH), prolactin, and oestradiol showed no striking differences between the exposed and the non-exposed subjects. Total and low density lipoprotein cholesterol (LDL cholesterol) levels did not significantly differ between the two groups but mean high density lipoprotein cholesterol (HDL cholesterol) levels were higher in the exposed group; this could be due to relatively high HDL cholesterol in two women exposed to zeranol or relatively low HDL cholesterol in three non-exposed men.