It is assumed that an effective human immunodeficiency virus type 1 (HIV-1) vaccine should be capable of eliciting neutralizing antibodies. However, even the best antibodies known to date lack neutralizing ability against a significant proportion of primary HIV-1 variants and, despite great efforts, still no immunogen is available that can elicit humoral immunity which is protective against infection or disease progression. We tested sera from 35 participants in the Amsterdam Cohort Studies on HIV-1 infection, who were all infected with HIV-1 subtype B and therapy-naïve at the time of sampling, for neutralizing activity against a panel of 23 tier 2-3 HIV-1 variants, with a minimum of five HIV-1 variants per subtype (A, B, C and D). Strong cross-clade neutralizing activity was detected in sera from seven individuals. Strikingly, sera from 22 of 35 individuals (63%) neutralized three or more of the six tier 2-3 HIV-1 subtype B viruses in the panel. There was a strong correlation between neutralization titre and breadth in serum. Indeed, the IC(50) of sera with strong cross-clade neutralizing activity was significantly higher than the IC(50) of sera with cross-subtype B activity, which, in turn, had a higher IC(50) than sera with the lowest neutralization breadth. These results imply that humoral immunity, at least in HIV-1 subtype B-infected individuals, is often subtype-specific rather than strain-specific and that the breadth of neutralization is correlated with the titre of neutralizing activity in serum. Considering the difficulties in designing a vaccine that is capable of eliciting cross-clade neutralizing activity, subtype-specific vaccines may be explored as an interesting alternative.