Blood gases and acid-base balance in 106 children suffering from cystic fibroais were studied during ten years. 68 of the patients survive, 38 died. Based on 670 examinations in surviving children /group A/ and 507 examinations in thoae who died /group B/, the prognostic value of hypercapnea was evaluated. The first examination showed hypercapnea in 18 per cent of the group A and in 43 per cent of B. During the first months of life high pCO2 values were found in both groups. In older children hypercapnea is an alarming sign showing the necessity of intensive care. It was proved practically in all examinations made in the affected children during the last month of their life. Metabolic alkalosis caused either by vomiting or by excessive loss of electrolytes in the sweat can be considered as un anfavorable complication being hypoventilation. It was found - either isolated or combined with respiratory acidosis - in 3 per cent of cases.