The median age of the HIV-1 uninfected partner was 29 years [interquartile range (IQR) 26–36], and 20 % were <25 years of age (Table 1). The HIV-1 uninfected partner was male in 67 % of the partnerships. Most couples (97.8 %) were married or cohabitating and had been living together for a median of 2.5 years [IQR 0.8–7.0] but had only learned of their HIV-1 serodiscordant status a median of 1 month [IQR 1–3] prior to the screening date. Over half of the couples (56.5 %) had no children and 64.8 % reported sex unprotected by condoms in the month prior to enrollment. A third of HIV-1 uninfected males were not circumcised. The median age of HIV-1 infected partners was 28 years [IQR 23–35] and 41.8 % had plasma HIV-1 RNA levels ≥50,000 copies/mL (median 4.6 log10 copies/mL, IQR 3.8–5.0). The median CD4 count was 436 cells/uL [IQR 272–638] and 41 % had a CD4 count >500 cells/μL. More than a third (34.7 %) of HIV-1 infected partners with CD4 counts >500 cells/μL were in partnerships that had a HIV-1 risk score ≥7 (Fig. 3). For twelve of the enrolled participants baseline plasma HIV-1 RNA levels were detectable even though the HIV-1 antibody test was negative, suggesting early HIV infection. Eligible couples who did not enroll had similar characteristics to enrolled couples.Table 1Baseline characteristics of enrolled HIV-1 serodiscordant couples in the Partners Demonstration Project (N = 1013)Median [IQR] or N (%)Age of HIV-1 uninfected partner29 (26–36) 18–2038 (3.8) 21–30534 (52.7) ≥ 31441 (43.5)Number of children within partnershipa 0 (0–1) 0572 (56.5) 1–2334 (33.0) ≥ 3107 (10.6)Not circumcised (male HIV-1 uninfected only)225 (33.1)Married/ cohabitating with study partnera 991 (97.8)Unprotected sex in month prior to enrollmenta 656 (64.8)Viral load of HIV-1 infected partner4.6 (3.8–5.0) < 10,000280 (28.1) 10,000–49,999301 (30.2) ≥ 50,000417 (41.8) aData obtained from HIV-1 uninfected partners Fig. 3Proportion of enrolled couples in each CD4 category, by risk score