Irritation and sensitization patch test studies were conducted using propylene glycol in an attempt to ascertain the nature of the cutaneous response to this commonly employed topical excipient. A total of 10 and 203 subjects completed standard irritation and sensitization protocols, respectively. A provocative use test was conducted on subjects reacting to propylene glycol. Results indicate that propylene glycol is at least a minimal irritant. Fleeting evidence suggestive of sensitization was observed during patch testing but was not substantiated upon provocative use testing. Interpretation of these results is presented; however, the nature of the cutaneous response to propylene glycol remains obscure.