Objective: To determine whether petrolatum or aqueous vehicles are more sensitive for detecting allergy to imidazolidinylurea (IU), diazolidinylurea (DU), and dimethylol dimethyl hydantoin (DM). The relationship of these allergens to formaldehyde sensitivity was also explored. Methods: Retrospective analysis of patients patch-tested by the North American Contact Dermatitis Group. All patients were simultaneously tested to seven allergens (formaldehyde, IU in petrolatum [pet], IU aqueous [aq], DU pet, DU aq, DM pet, and DM aq). Data were analyzed in pairs with various "gold standard" definitions of "true allergy" and adjusting for correlated data. Results: Reaction to at least one of the seven allergens occurred in 2,398 patients. In all cases except one (which just approached statistical significance), the petrolatum-based allergen was statistically significantly more sensitive than the same allergen in an aqueous base. Most of the patients allergic to the three preservatives were also allergic to formaldehyde, but most formaldehyde- allergic patients were not allergic to the IU, DU, or DM. Conclusion: Of these two vehicles, petrolatum is significantly more sensitive than an aqueous vehicle is for detecting allergy to IU, DU, and DM.