PURPOSE: Disenrollment behavior has important implications for the cost, quality, and continuity of care under Medicare Managed Care (MMC). Although disenrollment data are sometimes used as a quality indicator, uncertainty about how personal and plan characteristics influence disenrollment decisions complicates their use as a valid quality measure. METHODS: To determine how personal characteristics influence MMC disenrollment decisions, we analyzed data from the Medicare Beneficiary File, the US Census, and MMC plans. The study population consisted of MBs enrolled in one of 9 MMC Risk plans during a 6 month period, January through June 1998. We estimated logistic regression models with disenrollment (i.e. leaving a plan during the study period) as the dependent variable, and individual MB characteristics (age, gender, race, education, economic indicators) as independent variables, adjusting for plan characteristics. For disenrolling MBs we also estimated models using rapid disenrollment and disenrollment to fee-for-service (FFS) as dependent variables. RESULTS: Of 84,443 Medicare Beneficiaries (MBs) enrolled in one of 9 MMC risk plans in Connecticut, 4,102 (4.9%) disenrolled at least once during the 6 month study period, January through June, 1998. Of 4,022 MBs who disenrolled only once, 2,661 (66.2%) joined another MMC plan, while 1,361 (33.8%) returned to fee-for-service (FFS). 948 (23.6%) disenrolled rapidly (<90 days after joining). MBs who switched to another MMC plan had been enrolled longer than those who went back to FFS (Median 303 vs. 120 days, p < .001). In logistic regression analysis, adjusting for plan characteristics, disenrollment was associated with non-White race/ethnicity, age <75, and residence in areas with higher poverty rates and lower education levels. Among disenrollees, rapid disenrollment and disenrollment to FFS were associated with female gender, age ≥75, and residence in areas with higher poverty rates. CONCLUSION: Higher disenrollment among poorer, minority MBs suggests that these groups may have more problematic experiences with MMC, or may be more severely affected by coverage limitations. The higher rate of rapid disenrollment and return to FFS among older, poor, female disenrollees may be related to inadequate plan information at enrollment. Further research is needed do determine how patient characteristics influence disenrollment decisions, and whether MMC plans can improve member retention by paying more attention to the medical needs of specific groups and the information provided to enrollees.