<jats:p>Background: GNE myopathy is an ultra-rare muscle disease characterized by a reduction in the synthesis of sialic acid derived from pathogenic variants in the GNE gene. No treatment has been established so far. Objective: We evaluated the safety and efficacy of oral supplementation of aceneuramic acid in patients with GNE myopathy. Methods: This multicenter, placebo-controlled, double-blind study comprised genetically confirmed GNE myopathy patients in Japan who were randomly assigned into treatment groups of sialic acid-extended release (SA-ER) tablets (6 g/day for 48 weeks) or placebo groups (4:1). The primary endpoint of effectiveness was set as the change in total upper limb muscle strength (upper extremity composite [UEC] score) from the start of administration to the final evaluation time point. Results: Among the 20 enrolled patients (SA-ER group, 16; placebo group, 4), 19 completed this 48-week study. The mean value of change in UEC score (95% confidence interval [CI]) at 48 weeks was –0.1 kg (–2.1 to 2.0) in the SA-ER group and –5.1 kg (–10.4 to 0.3) in the placebo group. The least squares mean difference (95% CI) between the groups in the covariance analysis was 4.8 kg (–0.3 to 9.9; P = 0.0635). The change in UEC score at 48 weeks was significantly higher in the SA-ER group compared with the placebo group (P = 0.0013) in the generalized estimating equation test repeated measurement analysis. In one patient in the SA-ER group, who was found to be pregnant 2 weeks after drug administration fetal death with tangled umbilical cord occurred at 13 weeks after the discontinuation of treatment. No other serious adverse effects were observed. Conclusions: The present study indicates that oral administration of SA-ER tablets is effective and safe in patients with GNE myopathy in Japan.</jats:p>