This study reports the findings of a supplement marketed on the internet for prostate problems. The supplement was orally taken by a 60-year old man with divergent hormonal levels and who was surgically treated for gynaecomastia: development of abnormally large mammary glands in males. The supplement showed a strong effect in a yeast estrogen bioassay, expressing a yeast enhanced green fluorescent protein (yEGFP) upon exposure to estrogens. Using both NMR and a gradient liquid chromatographic time of flight mass spectrometric (LC/TOFMS) method, the response was shown to be caused by very high levels of diethylstilbestrol, known for causing gynaecomastia. The gynaecomastia was most probably caused by this orally taken 'natural' herbal supplement, as the patient hormonal levels also returned to normal again when stopping the use of it. This case demonstrates that physicians need to be aware of the use of supplements with illegal components that may be responsible for unwanted side-effects.