This study estimated the effect of mergers on health maintenance organization (HMO) premiums, using data on all operational non-Medicaid HMOs in the United States from 1985 to 1993. Two critical issues were examined: whether HMO mergers increase or decrease premiums; and whether the effects of mergers differ according to the degree of competition among HMOs in local markets. The only significant merger effect was found in the most competitive markets, where premiums increased, but only for 1 year after the merger. Our research does not support the argument that consolidation of HMOs in local markets will benefit consumers through lower premiums.
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