Purpose In oncology patients, hospital malnutrition is associated with a greater risk of morbidity and mortality. The objective of this study was to determine the prevalence of nutritional risk and the clinical and economic consequences of hospital malnutrition in oncology patients hospitalised in Spanish centres. Methods This was an observational, cross-sectional, multicentre study. The prevalence of nutritional risk was determined using the Nutrition Risk Screening ® -2002 (NRS ® -2002). Results Four hundred one oncology patients were included; 33.9 % (136/401) were at nutritional risk (NRS ® -2002 ≥ 3) at admission and 36.4 % (135/371) at discharge. On average, patients at nutritional risk were more elderly and had lower weights, body mass indices and arm and calf circumferences, as well as lower serum albumin levels than patients not at risk. Mean duration of hospitalisation and healthcare costs were greater in patients at nutritional risk at discharge (12.1 days; 95 % confidence interval (CI) 10.83–13.39) than in well-nourished patients (8.6 days; 95 % CI 7.86–9.40). Only a third of the patients at risk of malnutrition at discharge had received any kind of nutritional support. Conclusions This study shows that hospital malnutrition is a prevalent and undertreated condition in oncology patients that is associated with longer hospital stays and increased healthcare costs.