Because the incidence of human immunodeficiency virus (HIV) infection is difficult to measure directly, prevalence trends often serve to track epidemiologic changes. Adult HIV prevalence in open population cohort studies, however, reflects changes in incidence, population factors (migration, deaths, and aging), and survey coverage. Data from an open cohort in rural Uganda enabled estimation of the contribution of these factors to prevalence trends from 1989 to 2007. New infections within this cohort represented on average 44% of new prevalent cases per year. Other factors affecting changes in prevalence included migration and death. Migrants and mobile people (those who leave and return to the study area) are in a higher-risk group and thus can affect prevalence trends. Incidence of HIV infection among mobile people was 2-4 times greater than among stable residents. The importance of mortality is shown by the rise in prevalence from 6.8% in 2005 to 7.4% in 2007, which was accompanied by a fall in mortality among HIV-infected participants (8.7% of HIV-infected in 2005, 5.2% in 2006, and 4.3% in 2007). Assessing HIV epidemic trends through prevalence requires consideration of population factors. Measuring HIV incidence directly remains the most accurate measure of trends with which to monitor the effect of intervention activities and should complement strategies such as national prevalence surveys.