BACKGROUND: Modularity is an integral element of reverse total shoulder arthroplasty (RTSA). Glenosphere diameter is a key component, but its effect on revision rate is unknown. The aim of this study was to investigate the relationship between glenosphere size and revision rates in RTSA procedures. METHODS: Data from a large national arthroplasty registry were analyzed for the period April 2004 to 31 December 2019. The study population included all primary RTSA procedures using glenosphere sizes <38mm, 38-40mm, and >40mm. A subanalysis of glenosphere sizes for each of the three most commonly implanted prostheses and further analyses by patient age and gender were also performed. The rate of revision was determined by Kaplan-Meier estimates, with comparisons by Cox proportional hazard models. RESULTS: There were 28,817 primary RTSA procedures. Glenosphere sizes <38mm had a higher revision rate compared to 38-40mm glenospheres (HR =1.28 (95% CI) 1.11, 1.48), p<0.001) and >40mm sizes (HR=1.35 (1.15, 1.59),p<0.001). Males with <38mm and 38-40mm glenospheres had significantly higher revision rates compared to >40mm glenospheres (HR = 1.49 (95% CI 1.21, 1.83), p<0.001; and HR = 1.28 (95% CI 1.03, 1.58) p = 0.025, respectively). Females with <38mm and >40mm glenospheres had higher revision rates compared to females with 38-40mm glenospheres (HR 1.38(95% CI 1.14,1.68), p < 0.001 and HR 1.41(95% CI 1.06, 1.88), p = 0.019, respectively). For patients aged 65-74 years, glenospheres >40mm had a significantly lower revision rate than both the <38mm glenospheres (Entire Period: HR=0.59(95% CI 0.45, 0.76),p<0.001) and 38-40mm glenospheres (Entire Period: HR = 0.72(95% CI 0.54, 0.94), p = 0.017). For patients aged ≥75 years (n =14,622), <38mm glenospheres had a significantly higher rate of revision compared to 38-40mm glenospheres after 3 months (HR = 1.43(95% CI 1.09, 1.86), p = 0.009). Analysis comparing glenosphere sizes within each of the three most commonly implanted prostheses found that the Delta Xtend with 38-40mm glenospheres had higher revision rates compared to >40mm glenospheres (HR=1.49 (95% CI 1.14, 1.92), p=0.003). The SMR L1, 38-40mm glenospheres had a lower rate of revision compared to <38mm (HR= 0.50 (95% CI 0.37, 0.67), p < 0.001) and >40mm glenospheres (HR=0.60 (95% CI 0.43, 0.85), p=0.004). CONCLUSIONS: Glenospheres <38mm can be expected to increase revision rates in primary RTSAs. The optimum size of glenospheres is gender, age and prosthesis specific, with revision rates lower for females with 38-40mm glenospheres and lower for males with >40mm glenospheres.