Zearalenone (6-[10-hydroxy-6-oxo-trans-1-undecenyl]-beta-resorcylic acid-p-lactone), a mycotoxin produced in corn, is able to adopt a conformation which sufficiently resembles 17beta-oestradiol to allow it to bind to the oestrogen receptor in target cells of the body and exert (agonist) oestrogenic action. We adopted an analytical approach to isolate and quantify the mycotoxin and its derivatives using high-performance liquid chromatography and gas chromatography-mass spectrometry. In this study, the quantity of zearalenone and its congeners (alpha-zearalenol and beta-zearalenol) present in the plasma of patients with breast (n = 28) and cervical carcinoma (n=54) were compared with levels in patients presenting with other diagnoses (n = 26) and healthy volunteers (n = 24). There were no significant differences between the groups in the levels of zearalenone and its congeners, using analysis of covariance (0.2 < p < 0.6). These results suggest that the presence of this mycotoxin in blood does not indicate causal relationship between exposure to this exogenous myco-oestrogen and the subsequent biological effect in our study population but may be used as an indicator of exposure. The presence of zearalenone in the study groups is, however, of growing concern, due to the possible effects of cumulative long-term exposure of oestrogenic target organs to this compound.