HMO enrollees with limited English proficiency, and particularly those in fair to poor health, face communication barriers despite language assistance regulations. More than 1.3 million California HMO enrollees ages 18 to 64 do not speak English well enough to communicate with medical providers and may experience reduced access to high-quality health care if they do not receive appropriate language assistance services. Based on analysis of the 2007 and 2009 California Health Interview Surveys (CHIS), commercial HMO enrollees with limited English proficiency (LEP) in poorer health are more likely to have difficulty understanding their doctors, placing this already vulnerable population at even greater risk. The analysis also uses CHIS to examine the potential impact of health plan monitoring starting in 2009 (due to a 2003 amendment to the Knox-Keene Health Care Services Act) requiring health plans to provide free qualified interpretation and translation services to HMO enrollees. The authors recommend that California’s health plans continue to incorporate trained interpreters into their contracted networks and delivery systems, paying special attention to enrollees in poorer health. The results may serve as a planning tool for health plans, providing a detailed snapshot of enrollee characteristics that will help design effective programs now and prepare for a likely increase in insured LEP populations in the future, as full implementation of the Affordable Care Act takes place over the next decade.