The presence of resistance mutations in patients failing tipranavir or darunavir was examined at the national drug resistance database of the Spanish AIDS Research Network. Although mutations emerging during tipranavir and darunavir failures differed considerably, cross-resistance was found in up to half of the patients tested. Interestingly, mutation 54L, which is associated with tipranavir hypersusceptibility, was selected in half of the darunavir failures. Thus, resistance testing seems mandatory to ensure the benefit of the sequential use of these drugs.