The knowledge of which drug-resistant human immunodeficiency virus (HIV) genotypes are the most prevalent in a community may be helpful for designing the best salvage regimens. A total of 540 individuals on antiretroviral therapy attending 18 different outclinics in Spain were examined in a cross-sectional study conducted during June 2000. The overall rate of virologic failure (>50 HIV RNA copies/ml) was 54%. Among the subjects showing treatment failure, 79% harbored resistant HIV genotypes, 77% showed resistance to nucleoside analogues, 53% showed resistance to protease inhibitors, and 42% showed resistance to nonnucleoside reverse transcriptase inhibitors. Overall, 78.5% of individuals harbored HIV strains which showed resistance to two or more drug classes. Moreover, nucleotide substitutions causing broad cross-resistance among compounds within each drug family were quite common. These findings suggest that drug resistance mutations are very prevalent among subjects who have experienced several treatment failures. Therefore, facilitating the arrival of compounds belonging to new drug classes should be considered a priority.