Recent court cases in England and Wales have refocused attention on patients’ decision-making capacity to consent. Little is known about the prevalence of incapacity across specialities but decision-making capacity is likely to be overestimated by clinicians. The aim of this systematic review is to estimate the prevalence of incapacity to consent to treatment or admission in different medical and psychiatric settings, and compare the two. We conducted an electronic search following PRISMA principles and included 35 studies in psychiatric and 23 studies in medical settings. The 58 included studies revealed 70 data sets across all settings. For psychiatric settings the weighted average proportion of patients with incapacity was 45% (95% confidence interval (CI) 39–51%). For medical settings, the weighted average proportion of patients with incapacity was 34% (95% CI 25–44%). The two groups are not significantly different from each other in terms of the proportion of incapacity (p=0.92). A considerable number of medical and psychiatric patients lack capacity to make treatment and assessment decisions. Clinicians should be more alert to the possibility that their patients may lack decision-making capacity. Assessment of capacity should be frequent using the appropriate legal frameworks to act in the best interest of patients.