There are few reports of the association of nutritional status with in-hospital mortality of acutely ill medical patients in sub-Saharan Africa. This is a prospective observational study comparing the predictive value of mid-upper arm circumference (MUAC) of 899 acutely ill medical patients admitted to a resource-poor sub-Saharan hospital with mental alertness, mobility and vital signs. Mid-upper arm circumference ranged from 15 cm to 42 cm, and 12 (24%) of the 50 patients with a MUAC less than 20 cm died (OR 4.84, 95% CI 2.23–10.37). Of the 237 patients with a MUAC more than 28 cm only six (2.5%) died (OR 0.27, 95% CI 0.10–0.67). On logistic regression, the National Early Warning Score (NEWS), alertness, mobility and MUAC were independent predictors of in-hospital mortality. Mid-upper arm circumference is an independent predictor of the in-hospital mortality of acutely ill medical patients in a resource-poor hospital in sub-Saharan Africa.