Background: Congenital fibre type disproportion (CFTD) is an uncommon form of congenital myopathy in which type I fibres are consistently smaller than type 2 fibres in the absence of other histological abnormalities. Although most infants are severely affected, the extent of hypotonia and weakness is variable. We describe a case of CFTD presenting as slowly progressive hypotonia with respiratory failure. Case: A 5-month-old boy was transferred due to mild hypotonia and ventilator weaning failure. He was born at term following an uncomplicated pregnancy and delivery. In neonatal period, he showed normal muscle tone and reflexes in all extremities and had no dysmorphic feature except a high arched palate. However, he had gradually required feeding by gavage and ventilator support due to swallowing difficulties and recurrent aspiration pneumonia. Laboratory studies were unremarkable, including brain magnetic resonance imaging, muscle enzyme, electromyography and nerve conduction velocities, metabolic studies and genetic test for Prader-Willi syndrome, myotonic dystrophy, spinal muscular atrophy and myotubular myopathy. But generalized weakness had been progressive with failure of ventilator weaning and head lagging over time. Muscle biopsy revealed predominance and hypoplasia of type I fibres without other myopathic change and these findings were consistent with CFTD. TPM3 gene, one among genes associated with CFTD, had no mutation. At his age of 16 months, he presented slowly progressive hypotonia without deep tendon reflexes and continued to depend on ventilator, although he has shown advanced cognitive function. Conclusion: We describe a rare case with severe CFTD who developed progressive generalized weakness. In infants with progressive hypotonia, CFTD should be considered as one of the etiology and muscle biopsy can give important diagnostic clue.