We analyzed the didanosine (ddI) arm of the randomized, placebo-controlled Jaguar trial in order to define a genotypic score for ddI associated with virologic response. In this arm, 111 patients experiencing virologic failure received ddI in addition to their current combination therapy for 4 weeks. The impact of mutations in the reverse transcriptase gene on the virologic response to ddI was studied in univariate analysis. Genotypic score was constructed using step-by-step analyses first including only mutations associated to poorer virologic response (scored as +1), while secondarily, mutations associated to a better response (scored as -1) were also eligible. Eight mutations were associated with a reduced response to ddI, M41L, D67N, T69D, L74V, V118I, L210W, T215Y/F, and K219Q/E, and two mutations were associated with a better response, K70R and M184V/I. The best prediction of the virologic response to ddI was obtained with a composite score comprising mutations added and subtracted (set II, M41L + T69D + L74V+ T215Y/F + K219Q/E - K70R - M184V/I; P = 4.5 x 10(-9)) and by comparing that to only mutations added (set I, M41L + T69D + L74V + L210W + T215Y/F + K219Q/E; P = 1.2 x 10(-7)). Patients had a human immunodeficiency virus RNA reduction of 1.24, 0.84, 0.61, 0.40, and 0.07 log(10) copies/ml when they were ranked as having a genotypic score II of -2, -1, or 0 or 1 and 2 mutations or more, respectively. In conclusion, we developed and validated a genotypic score, taking into account mutations negatively and positively impacting the virologic response to ddI.