Hemophilia A (HA) is classified according to residual circulating FVIII (FVIII:C). Patients with FVIII:C 5%-40% are classified as mild HA; patients with FVIII:C 1%-5% are considered moderate; and patients with FVIII:C <1% are classified as severe HA. Patients with severe HA have two common mutations: Inversions of introns 1 and 22 of the F8 gene. The aim of this study is to report the case of a patient diagnosed with HA without any family history of the disease and treated in Fundação Hemominas in 2022. Male patient, NB, presented an extensive hematoma on the back after 24 hours of delivery. A blood count was performed, which reveals a hemoglobin value of 5 g/dL (reference value approximately 16 g/dL). Patient is admitted to the ICU where a blood transfusion and coagulometric tests are performed, indicating HA. The patient is referred to the pediatric hematology unit of Fundação Hemominas. Coagulation tests confirmed the diagnosis of severe HA. The patient was referred for molecular diagnosis of the disease. In addition, three members of the patient's family agreed to participate in the molecular diagnosis. The family members answered two questionnaires: 1) bleeding assessment score - ISTH (BAT-ISTH); and 2) arthralgia assessment. Peripheral blood was drawn from all participants in a citrate tube, and samples were processed for genomic DNA extraction. Genotyping of the intron 22 inversion mutation was performed by inverse PCR. The patient was positive for intron-22 inversion, confirming the diagnosis of severe hemophilia A. The patient's mother (34 years old), with a bleeding score of 3, without joint pain, was diagnosed with the same inversion and received genetic counseling when the report was handed over. The patient's aunt (30 years old), with a bleeding score of 0, without joint pain, had a negative result for intron inversion 22. The patient's grandmother, 59 years old, with a bleeding score of 1, who presented right knee pain, also had a negative result. Evaluation of the patient's three-generation pedigree suggests that the patient inherited the de novo mutation from his mother. In conclusion, molecular diagnosis of patients and their families is considered important for correct classification of disease severity and for the development of family studies with regard to genetic counseling of patients and women with mutated F8 genes. Additionally, molecular diagnosis is important to verify the de novo mutation rate in the population of patients with HA. We thank all participants, Fapemig, Hemominas, and the CGSH of the Ministry of Health.