This paper discusses an Index developed and used to measure accessibility to health services provided on an outpatient basis at the Kaiser Community Health Foundation, Cleveland, Ohio. The Index is based on four levels of urgency or severity of illness and on the waiting times to get entry into the system. The parameters of the Index, initially established by task force members, were tested on a sample basis against the judgments of Kaiser outpatient facility users and finally refined through the consensus-seeking Delphi technique using a panel representing Kaiser members, gatekeepers, physicians and administrators. The actual times required to gain accessibility in the various process stages and in the various departments were obtained through simulated requests for appointments, by monitoring telephone calls to the various gate-keeping stations, by direct observations, and by the use of a telephone traffic computer. The paper discusses the Index developed, the data collection methodologies, and the data analyses performed.