Implementation of preventive services in an HMO practice. Practice does not conform to guidelines unless the guidelines are specifically implemented and performance is monitored. Several examples of implementation in one health maintenance organization (HMO) are given. These include immunization for influenza and follow up of positive screening tests for colorectal and cervical cancer. Each implementation effort has required the development of systems, which in this HMO are automated. Several issues influencing implementation are discussed, including resource constraints and priorities for the allocation of new resources. Developers cannot expect that their guidelines will be incorporated into clinical practice. They must foster specific implementation plans. 