The objective was to examine the effect of BMI on the incidence of various infectious diseases in institutionalised, geriatric subjects. In a retrospective cohort study we analysed medical records of 619 patients aged 75 years and older (mean age 87·6 (sd 6·4) years) who were treated in a geriatric hospital in Vienna, Austria. The total incidence rate of infection in this population was 0·80 per person-year. The most frequent infections were urinary tract infections (0·30 per person-year), followed by infections of the lower respiratory tract (0·19 per person-year), diarrhoea (0·12 per person-year) and other infections (0·20 per person-year). Incidence risk ratios were obtained by a multiplicative Poisson regression model. There was a J-shaped curve in the incidence of infections recorded by BMI with a nadir at 27–28 kg/m2. Compared with the reference group with a BMI of 24–27·9 kg/m2, subjects with a lower BMI had a higher incidence rate of infections. The incidence risk ratios, adjusted for sex, age and chronic diseases, were 1·62 (95 % CI 1·21, 2·17) for those with a BMI of < 20 kg/m2 and 1·84 (95 % CI 1·40, 2·42) for those with a BMI of 20–23·9 kg/m2. However, also patients with a BMI of 28 kg/m2 and above had a higher incidence rate of infections, with an incidence risk ratio of 1·54 (95 % CI 1·07, 2·22). These results show that both underweight and obesity are associated with a higher risk of infections in institutionalised geriatric patients.