BACKGROUNDL: Pressure ulcers are common, costly and impact negatively on individuals. Pressure is the prime cause, and immobility is the factor that exposes individuals to pressure. International guidelines advocate repositioning; however, there is confusion surrounding the best method and frequency required. A pragmatic, multi-centre, open label, prospective, cluster-randomised controlled trial was conducted to compare the incidence of pressure ulcers among older persons nursed using two different repositioning regimens. Ethical approval was received. Study sites (n=12) were allocated to study arm using cluster randomisation. The experimental group (n=99) were repositioned three hourly at night, using the 30° tilt; the control group (n=114) received routine prevention (six-hourly repositioning, using 90° lateral rotation). Data analysis was by intention to treat; follow-up was for four weeks. All participants (n=213) were Irish and white, among them 77% were women and 65% aged 80 years or older. Three patients (3%) in the experimental group and 13 patients (11%) in the control group developed a pressure ulcer (p=0·035; 95% CI 0·031-0·038; ICC=0·001). All pressure ulcers were grade 1 (44%) or grade 2 (56%). Mobility and activity were the highest predictors of pressure ulcer development (β=-0·246, 95% CI=-0·319 to -0·066; p=0·003); (β=0·227, 95% CI=0·041-0·246; p = 0·006). Repositioning older persons at risk of pressure ulcers every three hours at night, using the 30° tilt, reduces the incidence of pressure ulcers compared with usual care. The study supports the recommendations of the 2009 international pressure ulcer prevention guidelines. An effective method of pressure ulcer prevention has been identified; in the light of the problem of pressures ulcers, current prevention strategies should be reviewed. It is important to implement appropriate prevention strategies, of which repositioning is one. © 2011 Blackwell Publishing Ltd.