A positive correlation between malnutrition and length of hospital stay has been shown previously. However, the impact of malnutrition on an incidence of nosocomial infections and their effect on the duration of hospitalization has not been established among paediatric patients in developed countries. We investigated the correlation between malnutrition, nosocomial infections and LOS in hospitalized children in our University Children's Hospital.We conducted prospective longitudinal cohort study during the period of 1 year. Nutritional status (body weight and length/height, body mass index) was measured on hospital admission and the existence and type of nosocomial infections were monitored and evaluated during hospital stay of all recruited children.367 paediatric patients were included into the study (girls 184, 50.1%; mean age 7.3 years (IQR: 1 month - 18 years). On hospital admission overall 47 patients (12.8%) were malnourished; moderate malnutrition (BMI -2 to -3 SDS) was present in 8.7% patients, severe malnutrition (BMI <-3 SDS) in 4.1%, and 29 (7.9%) patients were stunted ((body length) BL/(body height) BH < -2 SDS). Altogether 21 children (5.7%) experienced nosocomial infection. Malnourished patients acquired nosocomial infections significantly more often in comparison to well-nourished patients (11/47 (23.4%) vs. 11/318 (3.5%), respectively; p < 0.001). The LOS was significantly longer in children who were malnourished (12.3 vs. 7.3 days, respectively; p = 0.004), and by an identical number of days, the hospitalization was prolonged also in children who developed nosocomial infection (12.3 vs. 7.3 days, respectively; p = 0.004). Binary logistic regression showed that odds of acquiring nosocomial infections were 5 times higher (RR 5.1 (95% CI 2.276-11.485)) in children who were malnourished, and the risk increased with prolonged hospital stay.Malnutrition is an important risk factor for developing nosocomial infection, and they both independently increase LOS. However, what is the cause and what is the consequence in the vicious circle of malnutrition, nosocomial infection and LOS remains questionable. Nevertheless, the timely addressed malnutrition could prevent longer hospital stay and higher incidence of nosocomial infection.