Latanoprost is an F-series prostaglandin analog which has been approved for use as an ocular hypotensive drug.{1107} 5-trans Latanoprost is an isomer of latanoprost wherein the double bond between carbons 5 and 6 has been changed from cis (Z) to trans (E). The trans isomer of latanoprost occurs as an impurity of between 2-5% in most commercial preparations of the bulk drug product. The present compound was prepared primarily as an analytical standard for detection and quantitation of this impurity. From what can be inferred from the study of other trans isomers of F-type prostaglandins, the biological activity of 5-trans latanoprost is likely to be similar to that of the cis isomer. However, there are no specific published reports on the biological activity, in particular on the intraocular hypotensive activity, of 5-trans latanoprost.