There is currently little information concerning the prevalence of zeranol and taleranol in animal urine following metabolism of the naturally occurring Fusarium spp. toxins. An epidemiological study is described which involves four European Union control laboratories in which 8008 urine samples were screened for the presence of zeranol using a time-resolved fluoroimmunoassay (TR-FIA). Of these samples, 93.6% screened negative for zeranol. All samples testing positive for zeranol were then analysed with a confirmatory method. Based on the confirmatory results, the TR-FIA-positive samples were then categorized as false-positive, true-positive or 'equivocal' (zeranol/taleranol and the Fusarium spp. toxins detected). The true-positive samples represented only 0.05% of the total number of samples (n = 4). After statistical analysis, 170 of 174 equivocal samples proved to belong to a 'normal' population in which the amount of zeranol/taleranol could be related to the total amount of Fusarium spp. toxins through a linear regression with a 99% prediction interval. This suggested that the presence of zeranol in these samples might be due to in vivo metabolism of the Fusarium spp. toxins. The presence of zeranol in the four remaining 'outliers' might be attributable to zeranol abuse rather than to natural contamination. The results are of interest for control laboratories as they might provide an analytical tool to help distinguish between abuse and natural contamination in zeranol testing.