In this 1999 JAMA study, Drs. David Himmelstein, Steffie Woolhandler, Ida Hellander and Sidney Wolfe examined the performance of for-profit HMOs vs. nonprofit HMOs on 14 quality-of-care indicators. Compared to the nonprofits, for-profit HMOs performed worse on all 14 care indicators. After controlling for HMO model type, geographic region and method of data collection, the researchers found investor-ownership to be consistently associated with lower quality care.
Read "Quality of Care at Investor-Owned vs. Not-for-Profit HMOs" (pdf)