h Skin biopsy samples from 145 relapse leprosy cases and from five different regions in Brazil were submitted for sequence analysis of part of the genes associated with Mycobacterium leprae drug resistance. Single nucleotide polymorphisms (SNPs) in these genes were observed in M. leprae from 4 out of 92 cases with positive amplification (4.3%) and included a case with a mutation in rpoB only, another sample with SNPs in both folP1 and rpoB, and two cases showing mutations in folP1, rpoB, and gyrA, suggesting the existence of multidrug resistance (MDR). The nature of the mutations was as reported in earlier studies, being CCC to CGC in codon 55 in folP (Pro to Arg), while in the case of rpoB, all mutations occurred at codon 531, with two being a transition of TCG to ATG (Ser to Met), one TCG to TTC (Ser to Phe), and one TCG to TTG (Ser to Leu). The two cases with mutations in gyrA changed from GCA to GTA (Ala to Val) in codon 91. The median time from cure to relapse diagnosis was 9.45 years but was significantly shorter in patients with mutations (3.26 years; P ‫؍‬ 0.0038). More than 70% of the relapses were multibacillary, including three of the mutation-carrying cases; one MDR relapse patient was paucibacillary. There is no doubt about the efficiency of the currently used multidrug therapy (MDT) scheme for treatment of leprosy, as demonstrated by the strong decrease in disease prevalence since its implementation and the low number of reported relapse cases (18). However, there has been a scarcity of in-depth studies of relapse occurrences in recent decades (27). As is known, differentiating diagnosis of relapse and reactional states poses some difficulties in the field, being responsible for under-or overdiagnosis of both disease stages. This is important because undiagnosed relapse cases could contribute to continuing disease transmission. In addition, hardly any data on the contribution of emergence of drug-resistant strains of Mycobacterium leprae to leprosy relapses exist.Diaminodiphenylsulfone (DDS), also called dapsone, was the first drug to be effective against leprosy worldwide, and the first cases of resistance to dapsone were detected in 1964 and involved two single nucleotide polymorphisms (SNPs) in the gene folP1, located in codons 53 and 55 (8, 9, 14, 29). Rifampin is the key component of the standard multidrug regimen used for treatment of leprosy, and it has been shown that PCR-based DNA sequence analysis of the rpoB gene of M. leprae was in full concordance with rifampin susceptibility testing in the mouse footpad system (17, 30). In addition to dapsone and rifampin, ofloxacin is also used for leprosy treatment and is a quinolone with an action mechanism based on interaction with DNA gyrase (2); SNPs in gyrA and gyrB confer resistance or hypersensitivity to quinolones (15). Although there is not yet an official definition of multidrug resistance (MDR) in leprosy, in parallel with tuberculosis, we adopt this terminology when we encounter resistance to rifampin and one other drug of the standard M...