Objective: Agenesis of the diaphragm (AD) in adults is an extremely rare congenital malformation. To date, only a few descriptions of this pathology have been published in the scientific literature. The article presents a case of detection of a left-sided AD in a 25-year-old patient operated on for diaphragmatic relaxation. Additional examination methods, including echocardiography and ultrasonography of the abdominal organs and retroperitoneal space, did not reveal any other malformations in this patient. Due to the lack of plastic material of the appropriate size and type, plastic replacement of the diaphragm was postponed to the second stage of surgery. Keywords: Congenital malformations, anomalies of the diaphragm, aplasia of the diaphragm, agenesis of the diaphragm, agenesis of the diaphragm in adults, diaphragmatic hernia, plastic surgery of the diaphragm.