D-bifunctional protein deficiency (MIM 261515) is a rare hereditary disease causing severe biochemical abnormalities that are usually fatal within the first two years of life [1]. This disease is the most severe among a group of diseases known as peroxisomal fatty acid oxidation disorders. Infants with this disorder have severe mental and physical retardation, visual and hearing impairment and all develop seizures within the first few months of life. Brain scans of these infants typically find a range of severe abnormalities [2,3]. In Slovak population we first describe several still unpublished cases of D-bifunctional protein deficiency, which were subsequently confirmed by biochemical and molecular analysis. Sequence analysis of HSD17B4 gene revealed that the patient had two missense mutations N457D and R506C, both in heterozygote state, which lead to isolated defect of the dehydrogenase activity of D-bifunctional protein (D-bifunctional protein deficiency type II). In this family, we have successfully analysed of amniotic fluid by molecular and also by biochemical investigations and we found no abnormalities. Another child from this family died in toddler’s age (1.5 years) and it was thought that she probably died for cytomegalovirus (CMV) infection. Therefore, we successfully gained the DNA of mentioned deceased toddler and we revealed that she suffered for D-bifunctional protein deficiency, too.