The obstructive sleep apnea syndrome is characterized by recurrent episodes of complete and/or partial upper airway obstruction during sleep, usually associated with hipoxemia and hypercapnia. The prevalence is estimated to be around 2% and frequently it is not discussed with the parents in the health visit. If it is not properly identified it may carry serious consequences. We present a 2 year-old boy with clinical manifestations of heart failure, arterial hypertension and radiographic images of pulmonary edema recognized while a pre-tonsillectomy assessment for bilateral hypertrophic tonsils. He had history of snoring, disturbed sleep and sweating since the first year of life, with progression of the symptoms. The electrocardiography and echocardiography studies revealed pulmonary hypertension and dilation and hypertrophy of right cardiac chambers. The polysomnography recorded severe apneas and hypopneas with hipoxemia and CO2 retention. An adenoidtonsillectomy was done with favorable evolution and a slow improvement of cardiovascular abnormalities. It is necessary to be alert about the severe cardiovascular complications associated with the obstructive sleep apnea syndrome, and to include it in the differential diagnosis of small children with systemic or pulmonary hypertension.