Background:  A resurgent HIV epidemic among MSM has been identified in some developed countries. Nevertheless, reports from community-based cohorts of MSM showed stabilized incidence. We studied trends in HIV-1 incidence, sexual risk behavior, risk factors for HIV-1 seroconversion, and source of HIV-1 transmission among MSM in the Amsterdam Cohort Study from 1984 to 2009.

Methods:  Every 6 months, HIV^‑ MSM were tested for HIV-1 antibodies and completed a self-administered questionnaire. Seroconverters were questioned in detail on the assumed source of transmission. Poisson regression analysis was used to test for temporal trends in HIV-1 incidence and to identify risk factors for seroconversion. Incidence rate curves were modelled using restricted cubic splines. Trends in sexual risk behavior and changes in source of HIV-1 transmission were evaluated using GEE logistic regression models adjusting for correlations between intra-individual observations.

Results:  Of the 1627 MSM who where HIV negative at entry, 215 seroconverted during follow-up. The incidence of HIV-1 strongly decreased from 7.4/100 person-years in 1985 to 1.3/100 person-years in 1990, remained relatively stable around 1.0/100 person-years from 1991 until 1997, and slowly increased to 2.0/100 person-years by July 2009 (P =0.10). However, among MSM ≤30 years, the incidence increased significantly from 0.9/100 person-years in 1997 to 3.8/100 person-years in July 2009 (P<0.01). Insertive and receptive unprotected anal intercourse (UAI) increased from 1996 onwards, both with steady (SP) and casual partners (CP) (P <0.05). Multivariate analyses revealed receptive UAI with CP (strongest risk factor), >5 sexual partners, having gonorrhoea, and low education as independent risk factors for HIV-1 seroconversion. These risk factors for acquiring HIV-1 did not change over time. Today MSM are more likely to have contracted HIV-1 from casual partners. However, for MSM >30 years, trends are suggestive that from 2003 to 2008, a larger proportion of the infections are acquired from steady partners compared with 1984 to 1987 (odds ratio = 3.3, 95%CI 1.07 to 10.41).

Conclusions:  Followed by increases in sexual risk behavior from 1996 onwards, the HIV-1 incidence in the Amsterdam Cohort Study remained stable. However, among MSM ≤30 years the incidence increased. At present, new infections among young MSM seem to be mostly transmitted through unprotected sex with casual partners. Therefore, prevention measures should focus particularly on young MSM and their casual partners. Among older MSM, prevention should also focus on steady relationships.