To describe the characteristics of patients admitted to Pediatric Intermediate Care Units (PImCU) and to assess their illness severity trajectories. This prospective, observational, multicentre cohort study was conducted in seven French PImCUs between September 2012 and January 2014. All consecutive patients aged under 18 were included. The severity of illness was evaluated through the Paediatric Advanced Warning Score (PAWS), measured every 8 h for each patient. A latent class mixed model was used to identify severity trajectory classes. A total of 2868 patients were included. The median [interquartile range] age was 29 [5-103] mo and the median length of stay was 1 [1-3] d. The primary indication for admission was respiratory (44%). Almost 3% of the patients were subsequently transferred to a pediatric intensive care unit. Three severity trajectory classes were identified. In one class, comprising the largest proportion of patients, the PAWS was low on admission and did not change markedly over time. In this class, patients were older and had a shorter length of stay. The other two classes were characterized by a higher PAWS on admission and rapid or slow improvement. These patients were more severely ill, mostly due to respiratory failure. A large proportion of patients had a stable profile and no signs of severity which suggests that the stay in PImCU was not indicated but a part of these patients have remained stable perhaps because of the advanced monitoring and intensive nursing in these units. The study was registered with ClinicalTrials.gov Protocol, Identifier: NCT02304341, ClinicalTrials.gov .