We report 4 cases of photocontact dermatitis due to ketoprofen, a non-steroidal anti-inflammatory drug derived from propionic acid. We carried out a skin biopsy to examine the amount of ketoprofen in the eruptive skin. We investigated the cross-reactions between tiaprofenic acid, suprofen and ketoprofen by patch and photopatch testing. In case no. 1, 17 days after the discontinuance of Mohrus poultice (containing ketoprofen as an active ingredient), we detected ketoprofen 312.5 ng/g in the area of skin where the poultice was applied. All 4 cases reacted positively to the causative medicaments containing ketoprofen and ketoprofen 1% pet. 3 out of 4 cases reacted positively to tiaprofenic acid 1% pet. Only 1 case out of the ketoprofen and tiaprofenic acid positive cases reacted positively to suprofen 1% pet. Vehicles of patients' medicaments were negative in all 4 cases. We suspected that the key structure of the cross-reaction between ketoprofen and tiaprofenic acid and suprofen was the benzoyl radical.