Background and Objective: The clinical relevance of the detection of multiple respiratory viruses in acute bronchiolitis (AB) is not established. Our goal was to evaluate the effect of viral coinfections on the evolution and severity of AB. Methods: A retrospective observational study was conducted in a tertiary hospital in Spain, from September 2012 to March 2020. Infants admitted for AB, with at least one respiratory virus identified by molecular diagnostic techniques were involved. A comparison was made between single virus infection vs viral coinfections. The evolution and severity of AB were determined based on the days of hospitalization and admission to the Pediatric Intensive Care Unit (PICU). Results: Four hundred forty-five patients were included (58.4% male). Median weight was 5.2 kg (RIQ 4.2 – 6.5) and age 2.5 months (RIQ 1.4 – 4.6). A total of 105 patients (23.6%) were admitted to PICU. The Respiratory Syncytial virus (RSV) was the most frequent etiological agent (77.1%). A single virus was detected in 270 patients (60.7%) and viral coinfections in 175 (39.3%), of which 126 (28.3%) had two viruses and 49 (11%) had three or more viruses. Hospital length of stay (LOS) increased proportionally to the number of viruses detected, with a median of 6 days (IQR 4 – 8) in single infection, 7 days (IQR 4 – 9) in coinfection by two viruses and 8 days (IQR 5 – 11) in coinfection ≥ 3 viruses, (p=0.003). The adjusted Cox regression model showed that detection of ≥ 3 viruses was an independent risk factor for longer hospital LOS (HR 0.568, 95%CI 0.410 – 0.785). No significant association was observed between viral coinfections and the need for PICU admission (OR 1.151; 95%CI 0.737 – 1.797). Conclusions: Viral coinfections modify the natural history of AB prolonging the hospital LOS, proportionally to the number of viruses detected, without increasing the need for admission to the PICU.