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Mixed connective tissue disease (MCTD) is a frequently recognized rheumatic syndrome. A young female with this disease who showed marked restrictive ventilatory impairment, hypoxaemia, CO2 retention and marked weakness of the skeletal muscles of the neck and thorax is reported. Alveolar hypoventilation causing CO2 retention in blood is rare in MCTD. The patient improved with steroid therapy. The underlying mechanism of pulmonary function impairment in MCTD is discussed.