BACKGROUND: Turner syndrome is a chromosomal disorder which occurs in 1/2500 to 1/3000 female live births, but cases with mosaic karyotype of 45X/47XXX are rarely reported. CASE: A 13-year-old girl was newly diagnosed with Turner syndrome with karyotype of 45X/47XXX (40% of 45X cells and 60% of 47XXX cells). Probably because of the mosaicism, the clinical manifestation in this patient was less severe than that of typical Turner syndrome patients, and she was not referred until the age of 13 for short stature. Her bone age was only 8 years, and we started recombinant human growth hormone (1.0 U/kg/week) to facilitate growth. We planned to initiate estrogen-replacement therapy when her bone age reaches 12 years. CONCLUSION: This case reminds us that for girls with declining growth velocity, Turner syndrome should be considered, which might help with early diagnosis and treatment.