SUMMARY An elderly woman developed an itchy, weeping, erythematous, papular eruption, confined to the skin under her neoprene knee brace. Allergic contact dermatitis to diethylthiourea and to her neoprene knee brace were diagnosed by positive patch test reactions. Allergic contact dermatitis from thioureas may be underdiagnosed, as they are not tested as part of the standard patch test series. Clinicians are encouraged to consider this diagnosis in patients with reactions to synthetic rubber, especially neoprene.