T.R.U.E. Test is a ready-to-use patch test system, which contains 23 allergens and is the only Food and Drug Administration-approved source of allergens currently commercially available in the United States. Previously, allergens dispersed in either petrolatum or water and designed to be applied utilizing Finn Chambers were also commercially available in the United States. During a 5-year study at the University of Kansas Medical Center, 167 patients were patch tested using both Finn Chamber and T.R.U.E. Test methodologies. Discordant positive reactions were examined for clinical relevance. The Finn Chamber methodology was superior in detecting clinically relevant allergies to fragrance mix, balsam of Peru, and thiuram mix. T.R.U.E. Test performed somewhat better than the Finn Chamber in detecting relevant allergic reactions to nickel, neomycin, and methylchloroisothiazolinone/methylisothiazolinone. Neither T.R.U.E. Test nor Finn Chamber methodologies performed optimally in detecting relevant allergies to formaldehyde and carbamates. Practitioners limited to only the T.R.U.E. Test methodology need to be aware that relevant reactions to fragrances, rubber accelerators/pesticides (carbamates and thiurams), and formaldehyde may be missed with this system. (J Am Acad Dermatol 2001;45:836-9.)