BACKGROUND: Retention in care is key to improving HIV outcomes. The goal of this study was to describe 'churn' in patterns of entry, exit, and retention in HIV care in the United States and Canada. METHODS: Adults contributing ≥1 CD4 count or HIV-1 RNA (HIV-lab) from 2000 to 2008 in North American AIDS Cohort Collaboration on Research and Design clinical cohorts were included. Incomplete retention was defined as lack of 2 HIV-laboratories (≥90 days apart) within 12 months, summarized by calendar year. Beta-binomial regression models were used to estimate adjusted odds ratios (OR) and 95% confidence intervals (CI) of factors associated with incomplete retention. RESULTS: Among 61,438 participants, 15,360 (25%) with incomplete retention significantly differed in univariate analyses (P < 0.001) from 46,078 (75%) consistently retained by age, race/ethnicity, HIV risk, CD4, antiretroviral therapy use, and country of care (United States vs. Canada). From 2000 to 2004, females (OR = 0.82, CI: 0.70 to 0.95), older individuals (OR = 0.78, CI: 0.74 to 0.83 per 10 years), and antiretroviral therapy users (OR = 0.61, CI: 0.54 to 0.68 vs. all others) were less likely to have incomplete retention, whereas black individuals (OR = 1.31, CI: 1.16 to 1.49, vs. white), those with injection drug use HIV risk (OR = 1.68, CI: 1.49 to 1.89, vs. noninjection drug use), and those in care longer (OR = 1.09, CI: 1.07 to 1.11 per year) were more likely to have incomplete retention. Results from 2005 to 2008 were similar. DISCUSSION: From 2000 to 2008, 75% of the North American AIDS Cohort Collaboration on Research and Design population was consistently retained in care with 25% experiencing some changes in status or churn. In addition to the programmatic and policy implications, the findings of this study identify patient groups who may benefit from focused retention efforts.