Objectives To demonstrate the value of flexible bronchoscopy (FB) and bronchoalveolar lavage (BAL) when determining causes of lung infection in immunocompromised children; to investigate differences in causes and radiological features of lung infections following bone marrow transplantation (BMT) compared to other immunosuppressive conditions; to evaluate the reliability of radiological findings when predicting the pathogen. Methods We retrospectively evaluated 132 immunosuppressed children who underwent FB and BAL because pulmonary complications between January 1999 and May 2014 at the Hacettepe University Hospital Pediatric Pulmonology Unit. Two groups, Group I (n = 106) and Group II (n = 26), consisted of patients who had primary or secondary immunodeficiency and those who were immunosuppressed because BMT, respectively. Radiological findings before FB and macroscopic and microscopic findings of the procedure were evaluated. Results FB and BAL were diagnostic in 86/132 patients (65.1%) and the antimicrobial treatment changed for 75/132 patients (56.8%). The most common pathogen was bacteria (Streptococcus pneumoniae was the leading one). Bacteria were more frequent in Group I than Group II (P = .008). No significant difference in radiological findings between Groups I and II was found. Considering all patients, a significant association was detected between viral pathogens and radiologically interstitial infiltration and a ground-glass appearance (P = .003). However, no significant association was detected between bacterial and fungal pathogens and the radiological findings. Conclusion In immunosuppressed patients, FB and BAL should be evaluated early for clarifying the causative agents. Then, appropriate treatments can be utilised and the side effects and high cost of unnecessary treatment may be mitigated.