Hypertrophic pyloric stenosis is one of the most common congenital defects in infants. The incidence of infantile hypertrophic pyloric stenosis (IHPS) is approximately two to five per 1000 births per year. IHPS is characterized by a progressive hypertrophy of pyloric muscle. Etiopathogenesis of this disease is still unknown. Abnormalities of ganglion cells development, increased local production of growth factors, altered nitric oxide production and genetic factors have been implicated in the pathogenesis. IHPS typically presents with progressive projectile vomiting leading to dehydration that usually commences between third and eighth week of life. Severe infection, disease with increased intracranial pressure, and metabolic diseases should be considered in the differential diagnosis. The classic hypochloremic, hypokalemic, metabolic alkalosis is not a common findings which may be due to more early detection rate of IHPS. Ultrasonography (US) is the standard diagnostic procedure. Pyloric thickness of 3 mm or higher and length of 15 mm or higher by ultrasonography is widely accepted as diagnostic criteria for pyloric stenosis. Pyloromyotomy by Ramstedt has been the standard treatment of IHPS, however laparoscopic pyloromyotomy is becoming more common. The paper presents a case of a 21-days-old newborn diagnosed due to intensive regurgitation, weakness and apathy.