Sleep disordered breathing with or without nocturnal hypercapnic hypoventilation is a common complication of respiratory muscle weakness in childhood neuromuscular disorders. Nocturnal hypercapnic hypoventilation as a sign of respiratory muscle fatigue, portends a particularly poor prognosis. We aimed at identifying daytime predictors of sleep disordered breathing at its onset and sleep disordered breathing with nocturnal hypercapnic hypoventilation. Forty-nine children and adolescents (11.3+/-4.4 years) with progressive neuromuscular disorders were studied with inspiratory vital capacity, peak inspiratory pressure, arterial blood gases, polysomnography, and a ten-item symptoms questionnaire. Daytime respiratory function was prospectively compared with polysomnographic variables. Sleep disordered breathing was found in 35/49 patients (71%). Twenty-four (49%) had sleep disordered breathing with nocturnal hypercapnic hypoventilation. Inspiratory vital capacity and peak inspiratory pressure, but not symptom score, correlated with sleep disordered breathing and severity of nocturnal hypercapnic hypoventilation. Sleep disordered breathing-onset was predicted by inspiratory vital capacity<60% (sens. 97%, spec. 87%). Sleep disordered breathing with nocturnal hypercapnic hypoventilation was predicted by inspiratory vital capacity<40% (sens. 96%, spec. 88%) and PaCO(2)>40 mmHg (sens. 92%, spec. 72%,). Sleep disordered breathing can reliably be predicted from simple daytime respiratory function tests, which, if applied systematically, will improve recognition of nocturnal respiratory failure.