Thimerosal, a mercuric derivative of thiosalicylic acid, is a preservative used in several types of consumer products, including cosmetics, ophthalmic and otolaryngologic medications, and vaccines. As a result of allergic reactions and environmental concerns, its use has declined significantly during the past 2 decades. During a 5-year study at the University of Kansas Medical Center, 574 patients were patch tested to the North American Contact Dermatitis Group's standard allergen tray, which included thimerosal. The demographic data from thimerosal-allergic and nonallergic persons were compared. Statistically significant increases in thimerosal allergy were found among women, health care workers, secretaries, and cooks. Thimerosal-allergic persons were more likely to be allergic to neomycin, bacitracin, and tixocortol pivalate. Despite a high percentage of thimerosal-allergic patients in our test population, very few of these allergic reactions were found to be clinically relevant to the patient's current dermatologic condition. Using the Significance-Prevalence Index Number for thimerosal and contrasting this number with the Significance-Prevalence Index Number for other allergens on the North American Contact Dermatitis Group's standard tray, we propose that either ethyleneurea/melamine formaldehyde or bacitracin would be more useful than thimerosal as a commercially available screening allergen. (J Am Acad Dermatol 2001;45:23-7.)