Table 3. Ease of condom use with casual and steady partners, showing a trend towards ease of condom use with casual partners, and lower and inconsistent condom use with steady partners (Fisher’s exact test; p,0.001). Ease of Condom Use Casual (% and n) Steady (% and n) Never Sometimes Every Time doi:10.1371/journal.pone.0001954.t003 13.9% (34) 32.2% (79) 53.9% (132) 34.3% (84) 43.3% (106) 20.4% (50) (range 14–53 years). The median number of days performing sex work per week was 3 days (range 1–7 days) with a median of 2 clients see per day (range 1–10), with a median of 2 clients (range 0–30) seen in the previous week. Maintaining the Cohort Enrolment for the Phase 1 HIV-negative cohort began in September 2004 and the last participant was terminated in May 2007. The retention rate for the HIV-negative cohort was 86.1%. Despite the high mobility and low socioeconomic status of this population, only 13 (5.3%) of the HIV-negative cohort were terminated because they could not be traced. Other reasons for termination included participant relocation (n = 13), inability to adhere to the study schedule (n = 4), participant withdrew consent (n = 4), and death due to causes unrelated to study participation (n = 5). As part of the study protocol, the women in the cohort received regular clinical examinations. Key baseline characteristics are summarized in Table 4. On enrolment into the cohort, the HIV- negative cohort had a mean body mass index (BMI) of 31.0 kg/ 2 m . Of the 245 participants, 121 (49.4%) had a BMI greater than 30 kg/m2 (defined as obese) and a further 58 (23.7%) were 2 overweight with a BMI between 25 and 30 kg/m . Sixty-two participants (25.3%) had a normal BMI between 18.5 and 25 kg/ m2 and only five participants (2.0%) were underweight with a BMI less than 18.5 kg/m2. Despite the high prevalence of obesity in this cohort, hypertension was not common. The mean systolic blood pressure was 117 mmHg and the mean diastolic pressure was 76 mmHg. Only 29 (11.8%) of the women had a systolic pressure greater than 140 mmHg or a diastolic pressure greater than 90 mmHg. Moderate anaemia was relatively common in this population and 48 (19.7%) of the women had a haemoglobin less than 12 g/dL at study enrolment. On enrolment into the cohort and at 6-monthly intervals, these women underwent STI screenings for Trichomonas vaginalis, Neisseria Table 4. Baseline medical characteristics of the HIV-negative cohort (n = 245). Blood pressure (mmHg) Systolic Diastolic Body mass index (kg/m2) Hemoglobin (g/dL) Hematocrit (%) doi:10.1371/journal.pone.0001954.t004 Mean (SD) 117 (15) 76 (10) 31.0 (7.8) 12.7 (1.3) 37.3 (0.3) Range 90–200 55–110 17.2–54.5 7.7–16.1 25–46 CAPRISA 002 Study gonorrheae, Chlamydia trachomatis, Mycoplasma genitalium, syphilis, and Herpes simplex virus-2. Overall, 29.4% of women were infected with an STI at baseline. This percentage increases to 62.0% if bacterial vaginosis is included. Further, while none of the women were pregnant on enrolment, there were 32 pregnancies in the cohort for an incidence of 8.5 per 100 person-years (95% CI 5.6– 11.5). During this same period, there have been 5 deaths unrelated to study participation. Causes of death include: stab wound (1), clinically reported as idiopathic thrombocytopenia (1), and unknown (3). Finally, after 4784 monthly visits by 245 participants in this HIV-negative cohort, we identified 28 acute HIV infections. Twenty of the acute infections were among women who had self- identified as female sex workers and 8 of the acute infections were among women who had reported more than three partners in the preceding 3 months. The annual seroincidence after 390 person years was 7.2 per 100 person-years (95% CI 4.5–9.8). We investigated if the number of new HIV infections reduced over time, which could be due to women who were most at risk of getting infected seroconverting early during follow-up or women reducing their risky behaviour over time and in response to repeated risk-reduction counselling. There was a significant trend over time in the rate at which women seroconverted (p- value = 0.0283), and this is graphically illustrated in Figure 3. Discussion This paper describes the recruiting strategies, screening methods, enrolment procedures and retention techniques for a cohort of high-risk South African women, consisting largely urban female sex workers, a typically transient and mobile population that formed the basis for the CAPRISA 002 Acute HIV Infection study. We encountered numerous challenges over the course of establishing and administering the cohort and this paper aims to describe our strategies for overcoming these challenges. The first challenge was defining the inclusion and exclusion criteria for the cohort. The study design called for uninfected yet ‘‘high risk’’ women so that we could theoretically maximize the number of acute infections we were able to identify. As we began Figure 3. Percentage of acute HIV infections as a function of time in follow up (in days), showing a decrease in acute infections over time of follow up for the study. doi:10.1371/journal.pone.0001954.g003 PLoS ONE | www.plosone.org 5 April 2008 | Volume 3 | Issue 4 | e1954