There is limited information available about the prevalence and pattern of human immunodeficiency virus (HIV) drug resistance mutations (DRMs) among antiretroviral therapy (ART) experienced patients from northern India. Results of genotypic drug resistance testing were obtained from plasma samples of 128 patients, who had presented with clinical or immunological failure to treatment after at least six months of ART. Major DRMs associated with any of the three classes of antiretroviral (ARV) drugs, nucleoside reverse transcriptase inhibitors (NRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI) and protease inhibitors (PI), were seen in 120 out of 128 patients (93.8% prevalence). NRTI and NNRTI DRMs were each seen in 115/128 (89.8%) patients, with M184V, M41L, D67N and T215Y being the most frequent among NRTI associated mutations, and K103N, G190A, Y181C and A98G among NNRTI associated ones. PI DRMs were observed in 14/128 (10.9%) patients, with L10I, V82A and L89V being the commonest. These results present a high prevalence of DRMs among ARTexperienced patients from northern India with clinical or immunological failure of therapy. It emphasizes the need for regular testing of plasma samples of such patients for DRMs in order to detect and replace a failing regimen early, and also the use of HIV drug resistance genotyping of ART naive individuals prior to initiating first line ART for possible transmitted resistance. It is very important to enhance the access of patients to ARV drugs so that their compliance could be improved and hence development of DRMs be minimized.