Interest in 12-hour nursing shifts has been renewed in response to demands for improved cost-effectiveness in the NHS, but the effects of this shift on the delivery of patient care have been unclear. This paper describes the results of a repeated-measures study of 10 wards, using activity analysis to describe patterns of care under an 8-hour compared to a 12-hour shift system. Significant reductions in the amount of direct patient care were found under the 12-hour shift, with corresponding increases in unofficial work-breaks. It is suggested that these findings, which were consistent over all study wards and throughout the whole 12-hour day, demonstrate a "pacing" effect by nurses who face 12 hours on duty. Such a detrimental effect should be a major consideration when coming to any decision to implement a 12-hour shift.