Reported herein are 13 borderline lepromatous (BL) or subpolar lepromatous (LLs) patients who presented with or developed delayed-type hypersensitivity (DTH) reactions after initiation of antibacterial therapy, but who subsequently developed erythema nodosum leprosum (ENL), the DTH to ENL group. During the same time, three LLs patients had ENL followed by relapse-associated DTH, a significant (p < 0.05) difference in sequence of the two conditions. The DTH to ENL group had statistically significant higher biopsy indexes at the time of diagnosis of the DTH reaction compared with two DTH control groups, 7 multibacillary patients presenting with DTH reactions and 15 BL or LLs who developed DTH reactions after starting treatment but had no ENL. DTH-associated histologic changes were less well developed in the DTH to ENL group than in either of the two control groups. In the DTH to ENL group, 77% required prednisone in addition to thalidomide to achieve a complete remission in contrast to only 10% of 21 ENL clinical controls. In the DTH to ENL group, the classical histologic ENL pattern was present in only 31% of these patients, in contrast to 88% of 33 ENL histologic controls. In 9 of 9 of the DTH to ENL patients studied, after the ENL remitted, Mycobacterium leprae-sonicate-stimulated lymphocyte transformation tests gave stimulation indexes within the range of our tuberculoid (TT) and borderline tuberculoid (BT) patients, in contrast to absent responses in 6 ordinary, longterm-treated patients who had had ENL.