There is a need for consensus on the quantity of data that must be available in a computer-based information system of a health care organization. In this paper we take up the issue of defining the data content of an information system and introduce the concept of Essential Data Sets with an explicit methodology which was applied to define a data set for the Maternal Health Services program. A key step in the method was a recognized technique used in systems development process called data modelling, in this case infological modelling, by an interdisciplinary group. A preliminary set of 86 data elements was identified and it provided the foundation for development of an application software for discussion and a real-world testing framework. The acceptability of the data set was tested in a laboratory perspective by retrospective data entry from records of 94 pregnant women registered at a maternal health care center in Sweden. Data from a total of 1,318 prenatal visits, an outcome visit, and a postnatal visit for each woman was entered into a computer using the software, with no loss of information. Thus, in a short-term perspective the acceptability of the data set was demonstrated. The software has since been implemented for pilot prospective studies at sites in India and Sweden. The use of a common data protocol is an essential foundation for patient outcome research, especially as the trend of health care management has changed from a "process of care" orientation to an "outcome of care" orientation.