Nonoxynol-9 (N-9) containing spermicides and other N-9 containing products are commonly used as lubricants during rectal intercourse. We have previously demonstrated that rectal application of N-9 products in mice can cause exfoliation of epithelial cells, increasing the probability of infection by HSV-2. To determine if N-9-containing products would have a similar effect on the rectal epithelium in humans, the application of K-Y®Plus and ForPlay®, both over-the-counter (OTC) N-9 products, were compared to the application of two formulations, carrageenan and methyl cellulose, that do not contain N-9. The effects of each formulation were evaluated in 4 human participants. Light and electron microscope examination of rectal lavage specimens collected 15 min post application of N-9 products revealed the presence of sheets of epithelium. Each sheet contained hundreds of epithelial cells that included columnar and goblet cells, varieties of cells typical of rectal epithelial morphology. Sheets of epithelium were not observed in rectal lavage specimens collected 8 to 12 hr post N-9 product use or in either of the timed lavages involving non-N-9 containing formulations. In addition, no sheets of epithelial cells were observed in the baseline lavage specimens. We conclude that the rectal use of N-9-containing products causes a rapid exfoliation of extensive areas of the rectal epithelium. Exfoliation of the epithelium is no longer observed at 8 hr. It is reasonable to assume that the loss of the protective epithelium would render a person more at risk for infection by HIV and other sexually transmitted pathogens. We, therefore, caution against the use of N-9-containing products during rectal intercourse.