OBJECTIVES: To evaluate a multidisciplinary team (MDT) case management intervention, at the individual (direct effects of intervention) and practice levels (potential spillover effects). DESIGN: Difference-in-differences design with multiple intervention start dates, analysing hospital admissions data. In secondary analyses, we stratified individual-level results by risk score. SETTING: Single clinical commissioning group (CCG) in the UK's National Health Service (NHS). PARTICIPANTS: At the individual level, we matched 2049 intervention patients using propensity scoring one-to-one with control patients. At the practice level, 30 practices were compared using a natural experiment through staged implementation. INTERVENTION: Practice Integrated Care Teams (PICTs), using MDT case management of high-risk patients together with a summary record of care versus usual care. DIRECT AND INDIRECT OUTCOME MEASURES: Primary measures of intervention effects were accident and emergency (A&E) visits; inpatient non-elective stays, 30-day re-admissions; inpatient elective stays; outpatient visits; and admissions for ambulatory care sensitive conditions. Secondary measures included inpatient length of stay; total cost of secondary care services; and patient satisfaction (at the practice level only). RESULTS: At the individual level, we found slight, clinically trivial increases in inpatient non-elective admissions (+0.01 admissions per patient per month; 95% CI 0.00 to 0.01. Effect size (ES): 0.02) and 30-day re-admissions (+0.00; 0.00 to 0.01. ES: 0.03). We found no indication that highest risk patients benefitted more from the intervention. At the practice level, we found a small decrease in inpatient non-elective admissions (-0.63 admissions per 1000 patients per month; -1.17 to -0.09. ES: -0.24). However, this result did not withstand a robustness check; the estimate may have absorbed some differences in underlying practice trends. CONCLUSIONS: The intervention does not meet its primary aim, and the clinical significance and cost-effectiveness of these small practice-level effects is debatable. There is an ongoing need to develop effective ways to reduce unnecessary attendances in secondary care for the high-risk population.