PURPOSE: Collecting daily information on a series of similar, recurring events such as menstrual bleeding, headaches, or insulin levels using paper instruments is subject to problems such as missing or incorrectly recorded data, and retrospective data entry. The authors are developing and evaluating electronic data collection using hand-held personal computers (H/PC) for a variety of health-related applications, including tracking premenstrual syndrome (PMS), fertility awareness, and headaches, to improve accuracy and timeliness of data collection.METHODS: ProCycle is a prototype electronic diary for collection of daily data on menstrual bleeding, medications, and health symptoms. In a 3-month pilot test in 25 regularly cycling women, we compared its performance with a paper calendar regarding missing and incorrect data, data entry lag, data cleaning time, and users' preferences with respect to factors such as remembering to enter data, convenience, and overall preference. Additional programs for PMS, fertility, and headaches are being field tested on subjects from the Boston and New York areas, comparing performance with paper versions.RESULTS: In the pilot test, missing data occurred less frequently with ProCycle than with paper, particularly for any days with missing symptoms (4% vs. 35%, p < 0.05). ProCycle did not permit any data recording mistakes such as circling contradictory responses (e.g., bleeding and no bleeding), compared with incorrect data on 13% of paper calendars. Data entry/cleaning time was 81% lower for ProCycle. 70% of users preferred ProCycle overall, compared with 9% preferring paper (p < 0.01).CONCLUSIONS: Although the initial cost of the H/PC is significantly higher than paper, there are no recurring charges for printing or data entry, and data cleaning is minimal. Electronic instruments on an H/PC provide an efficient, accurate method of data collection, applicable to a number of areas of health-related research involving daily data collection.