A toxicologic and dermatologic review of 4-cyclopentadecen-1-one when used as a fragrance ingredient is presented. 4-Cyclopentadecen-1-one is a member of the fragrance structural group macrocyclic ketones and derivatives. The fragrance ingredient described herein is one of 11 structurally diverse C15, C16, and C17 compounds that include 3 saturated and 8 unsaturated ketones. For the latter, the double bond is not adjacent (in conjugation with) to the ketone group. This review contains a detailed summary of all available toxicology and dermatology papers that are related to 4-cyclopentadecen-1-one and is not intended as a stand-alone document. Available data, including physical properties data, were evaluated then summarized. A safety assessment of the entire macrocyclic ketones and derivatives will be published simultaneously with this document; please refer to Belsito et al. (2011) for an overall assessment of the safe use of this material and all macrocyclic ketones and derivatives in fragrances. Belsito, D., Bickers, D., Bruze, M., Calow, P., Dagli, M., Fryer, A.D., Greim, H., Hanifin, J.H., Miyachi, Y., Saurat, J.H., Sipes, I.G., this issue. A toxicologic and dermatologic assessment of macrocylic ketones and derivatives when used as fragrance ingredients.