Abstract Background Few studies have investigated the effects of waiting room communication strategies on health-care behavior. Objective We aimed to determine the effect of a waiting room communication strategy, designed to raise awareness of potential harms of unnecessary imaging, on lumbar imaging rates in the emergency department (ED). Methods We conducted a controlled experimental study with a replicated time series design. The design included a 6-week run-in time. Following this there were alternating 1-week intervention and control periods. The intervention group received a communication strategy describing the potential harms of unnecessary imaging for low back pain, shown on a 55” LCD screen positioned in the ED waiting room. The communication strategy was designed by a creative innovation agency and included five digital posters and a patient leaflet. The control group received standard messaging for the waiting room at the time, shown on the same 55” LCD screen, and access to the patient leaflet. The primary outcome was the number and proportion of people presenting to ED with low back pain who received at least one lumbar imaging test, measured using routinely collected ED data. Secondary patient-reported outcomes (patient satisfaction and awareness of campaign messages) were collected from a sample of people presenting for any condition who responded to a text-message-based survey. Results For the imaging outcome, 337 people presenting to ED with low back pain were included over a 4-month period (intervention n = 99; control n = 238). All had available data on lumbar imaging. Use of lumbar imaging was 25% in those exposed to the communication strategy [95% confidence interval (CI) = 18% to 35%] compared with 29% in those exposed to the standard waiting room messaging [95% CI = 23% to 35%; odds ratio (OR) = 0.83, 95% CI = 0.49 to 1.41]. For the patient-reported outcomes, 349 patients presenting to ED for any condition responded to the survey (intervention n = 170; control n = 179; response rate = 33%). There was uncertain evidence that the intervention increased awareness of the communication strategy leaflet (OR = 2.00, 95% CI = 0.90 to 4.47). Other measures did not suggest between-group differences in patient satisfaction or awareness of the campaign messages. Conclusion A communication strategy displayed in the ED waiting room may slightly reduce the proportion of patients with low back pain who receive lumbar imaging, although there is uncertainty due to imprecision. The campaign did not appear to increase awareness of campaign messages or affect patient satisfaction in a sample of patients presenting to the ED for any reason. Larger studies should investigate whether simple, low-cost waiting room communication strategies can raise awareness of unnecessary healthcare and influence health-care quality. Trial registration ACTRN12620000300976, 05/03/2020