BACKGROUND: LAMP3 encodes a lysosomal membrane protein associated with lamellar bodies and has recently been proposed as a candidate gene for childhood interstitial lung diseases (chILD). Here, we identified two LAMP3 variants in a proband with chILD and performed functional validation of these variants as well as the previously reported variants to demonstrate the role of LAMP3 in pathology. METHODS: LAMP3 variants were identified by exome sequencing. Ex vivo studies included mRNA analysis from nasal brushing and lung tissue and immunohistochemistry from lung biopsy. In vitro functional analyses in the A549 cell line included immunofluorescence staining and expression analysis of LAMP3. Interactions between LAMP3 and the surfactant protein (SP)-B and SP-C were evaluated by co-immunoprecipitation. FINDINGS: Two heterozygous LAMP3 variants (Y302Qfs∗2 and T268M) were identified in a 15 year old boy with chILD. LAMP3 mRNA revealed that the frameshift variant resulted in nonsense-mediated mRNA decay. Reduced LAMP3 expression was confirmed in the patient's lung tissue. Functional studies of the T268M and the previously reported G288R variant revealed reduced levels of the mutant proteins. In addition, impaired N-glycosylation and protein instability were demonstrated with the T268M variant. Finally, we provided evidence for an interaction between LAMP3 and SP-B and SP-C, revealing a direct link between LAMP3 and surfactant metabolism. INTERPRETATION: LAMP3 bi-allelic variants leading to LAMP3 dysfunction emerges as a cause of chILD associated with a heterogeneous phenotype that remains to be further defined. The close links between LAMP3 and surfactant metabolism could explain the pathophysiology of this genetic disease. FUNDING: No specific funding.