To evaluate the reproducibility of patch test results, duplicate patch test series comprising 12 allergens were simultaneously applied to either side of the back in 63 patients by the use of a newly developed patch test system, TRUE Test. When a positive test result occurred on only one side of the back, it was regarded as nonreproducible. For evaluation of the concordance of test reactions between the TRUE Test and the Finn Chamber test, the same 12 allergens mixed in petrolatum were also applied to the upper back according to the Finn Chamber technique. The reproducibility of TRUE Test results was compared with the reproducibility of the Finn Chamber test in 76 other patients. The concordance of positive reactions between the two tests was 57%, somewhat lower than in recently published studies. The percentage of nonreproducible positive results was more than twice as high with Finn Chambers, (22 of 58 positive reactions, or 37.9%) as with the TRUE Test (5 of 28 reactions, or 17.9%). However, this difference in reproducibility in the two test systems was not significant as determined by the x² test (p<0.05).