Congenital sialidosis is a rare disease resulting from the absence of neurominidase and presenting with hydrops fetalis, hepatosplenomegaly, dysmorphic features, vacuolated lymphocytes and extensive vacuolation of the connective tissue. Elevated levels of sialooligosaccharides in the urine is characteristic. We describe a newborn baby with congenital sialidosis and discuss the difficulties in reaching the diagnosis.