AbstractThis paper discusses an unusual etching of unlike-sex conjoined twins of the syncephalus type dated 1547. The authenticity of the case is discussed in terms of the biological plausibility of such twins and in the light of mediaeval understanding of the twinning process. Unlike-sex syncephalic twins may occur as a result of dispermic fertilisation, or be an erroneous depiction of an asymmetrical anomaly of the external genitalia of a like-sex pair, mimicking the genitals of the other sex. On the other hand, the Löwen etching conforms with mediaeval beliefs that conjoined twins are divine punishment and therefore were used for ecclesiastical and perhaps commercial purposes. The historical context of the drawing, in which the relation between religion, media, and medicine satisfied the eagerness of the public for curios of Nature, is no longer valid. Thus the Löwen etching opens a window to the past from which modern medicine can be appreciated. Twin Research (2000) 3, 185–188.