Estimation of enrollment penetration of target employee' groups, as a percent of total employees, is a critical task in the planning and development process of HMOs. A heuristic process to estimate penetration rates of employee groups is described. Next, a multivariate linear statistical model is proposed and the parameters of the proposed model are estimated on the basis of a data base comprising 83 employee groups for which penetration rates have been estimated using the heuristic process. The linear multivariate model is found to be statistically significant for both the singles and family (marrieds) penetration rates. The proposed model simplifies the method for estimating penetration but has not been tested against actual data.