We were given the opportunity to design and implement a general data processing system to accommodate several different epidemiologic studies to be conducted by a new research group. A survey of 15 operating data centers was conducted in preparation for undertaking the design and development of our system. The results of the survey indicated that data processing activities can be classified, both conceptually and operationally, into three modules: data recording and data entry, data management, and data analysis, and that the data management functions were those amenable to generalization. Based on our survey and the varying needs of our studies, we selected a "mixed" hardware environment, using both a computer center mainframe and microcomputers. We created the systems using commercially available software, including a mainframe database manager and mainframe statistics packages, microcomputer data entry software, and a communications package to link the two environments. Our strategy was to buy software, when possible, rather than to build custom programs, and to let software tools govern hardware needs. Hardware independence, price, and functional capability directed our software choices, while hardware selection was constrained most importantly by available software, then by budget, by available computing resources, and finally by the marketplace. The system has been used successfully in three studies differing in design, size, data collection locale, and rate of data accrual.