Research Question Did a 15-min patrol delivery over 1 day reduce serious violent crime in large hot spots (mean size = 2 km × 2 km), without displacing such crimes to nearby areas? Data We tracked daily official crime reports in a sample of 21 high-crime Bedfordshire (UK) Lower-layer Super Output areas (LSOAs). We measured time spent by two-person police foot patrols in those areas with daily GPS data from handheld devices given to officers working on overtime. We also counted proactively initiated arrests. Methods We used a crossover randomised controlled trial on the 21 “hot spot” LSOAs, each of which was randomly assigned daily to be either in a treatment condition of 15-min of patrol (as one of seven each day) or a control condition of no patrol (as one of 14 each day) for each of 90 days. We used an intention-to-treat framework to analyse the impact of patrols on the outcome measures overall, on consecutive days of assignment to the same condition, and in 100-m ‘buffer’ zones around each hot spot. Findings We found that on treatment days the hot spots had 44% lower Cambridge crime harm index scores from serious violence than on control days, as well as 40% fewer incidents across all public crimes against personal victims. Statistically significant differences in lower prevalence, counts and harm of both non-domestic violent crime and robbery and other non-domestic crimes against personal victims were also found. We found no evidence of either displacement of serious crime into a 100-m buffer zone, nor any evidence of residual deterrence on no-patrol days following patrol days. We did find evidence of a cumulative effect: the largest differences in crime harm on control days were found in treatment days that came after 3 days of consecutive patrol in the same LSOA. Conclusions Even minimal amounts of foot patrol can prevent serious violent crime across a large area, and repeated patrols over several days help even more. Our findings suggest that, to reduce both violent and other forms of crime, uniformed officers need to patrol hot spots for short amounts of times on consecutive days.