BACKGROUND: Mutation of the sialidase gene is the etiology of sialidosis, a storage disorder with a plethora of systemic manifestations ranging from ocular abnormalities, bone pathologies, and ataxia (sialidosis type I) to mental decline and infantile death (sialidosis type II). Non-immune hydrops fetalis and isolated ascites are the most severe forms of sialidosis type II that manifests itself prenatally.CASE REPORT: For the first time, we report congenital sialidosis with homozygous pathogenic deletion of the entire gene in a Greek neonate with hydrops fetalis, isolated ascites, central nervous system hypoplasia, and lethal progression. Genetic characterization of the patient showed one previously unreported deletion in the NEU1 gene.CONCLUSION: Sialidosis type II should be considered in the differential diagnosis of neonatal hydrops fetalis of no immune causality or isolated fetal ascites. Genetic studying of the patient and the family by carrier detection is crucial to prevent missed diagnoses, while genetic counseling for following pregnancies is imperative. HIPPOKRATIA 2019, 23(4): 169-171.