Abstract Background Cataract surgery is one of the most frequently performed types of surgery. Most patients suffer from bilateral cataract and while cataract surgery of only one eye is effective in restoring functional vision, second-eye surgery leads to further improvements in health-related quality of life, and is cost-effective. At present, most patients undergo cataract surgery in both eyes on separate days as recommended in national guidelines, referred to as delayed sequential bilateral cataract surgery (DSBCS). An alternative procedure involves operating both eyes on the same day, but as separate procedures, known as immediately sequential bilateral cataract surgery (ISBCS). The aim of this study is to evaluate the effectiveness and costs of ISBCS compared to DSBCS, in order to test the hypothesis that ISBCS is non-inferior to DSBCS in terms of effectiveness and superior to ISBCS in terms of cost-effectiveness. Methods/design Multicenter non-inferiority randomised controlled clinical trial. Patients (18 years or older) with bilateral cataract and an indication for bilateral cataract surgery with an expected uncomplicated intraoperative and postoperative course are included in the study. Patients are randomly assigned to either ISBCS or DSBCS. The primary endpoint is the proportion of patients with a refractive outcome in the second eye within 1.0 dioptre from the target refraction, at 4 weeks after surgery. Secondary outcomes include corrected and uncorrected distance visual acuity, complications, patient reported outcomes (PROMs), cost-effectiveness, and budget impact. Follow-up visits are planned at 1 week after first-eye surgery and 4 weeks after second-eye surgery. At 3 months after first-eye surgery, the occurrence of complications is checked and patients fill in a final questionnaire. Discussion This study protocol describes the design of a multicenter non-inferiority randomised controlled trial. Current studies on ISBCS often lack information on safety regarding refractive outcomes. In addition, ...