Effects of increased cost-sharing in Washington State's Basic Health Plan (BHP) were assessed among adult BHP beneficiaries (N=14,515) and age-sex-residence matched controls enrolled in Group Health Cooperative. The BHP enrollees had higher disenrollment than controls before and after cost-sharing increases, but disenrollment did not change with increased cost-sharing. Basic Health Plan enrollees' out-of-pocket-costs increased 100% in two years, compared with 42% for controls. Out-of-pocket costs for BHP enrollees with diabetes increased 61% (from $675 to $1,086), while the 90th percentile increased 74%, from $1,358 to $2,365. Basic Health Plan enrollees had somewhat fewer visits than controls after cost-sharing increases, but total costs, timeliness of glucose monitoring, and glycemic control were unaffected. Cost-sharing changes increased out-of-pocket costs for BHP enrollees without affecting total costs, disenrollment, or diabetes quality of care indicators. The predominant effect of increased cost-sharing was to increase costs for low-income workers, particularly those with chronic disease.