A cross-sectional study of shiftwork-related problems was carried out among health care workers at hospitals of Shiraz University of Medical Sciences, Islamic Republic of Iran. Data on personal details, shift schedule and adverse effects of shiftwork were collected from 432 randomly selected subjects by questionnaire. Sleep, social and subjective problems were more prevalent in shiftworkers than day workers. Irregular shiftwork schedules caused more social and subjective problems, as well as work dissatisfaction. Voluntary selection of shiftworking produced fewer health problems.