Diethylthiourea, like other thioureas, is often used by the rubber industry and in the manufacture of neoprene. We present a patient who suffered allergic contact dermatitis to diethylthiourea in a neoprene wader and who required admission to hospital and systemic treatment. We review the literature on allergy to diethylthiourea. Thioureas are not included in the standard GEIDAC (Spanish Contact Dermatitis Research Group) battery of patch tests. In these cases, it is necessary to use a special battery of rubber allergens, which includes thiourea compounds, for diagnosis of the disease and to ensure that cases of contact allergic dermatitis to thioureas do not go undiagnosed.