Purpose: The effect of growth hormone replacement on Chiari I malformation (CIM) associated with growth hormone deficiency is not clear. Case reports: Two patients are presented, who were found to have CIM and growth hormone deficiency. While on hormone replacement therapy, both experienced disease progression with development of syringomyelia and required surgical intervention. Conclusion: Growth hormone replacement for CIM associated with growth hormone deficiency does not uniformly halt or reverse syrinx progression. If a trial of hormone replacement is attempted, patients should be followed closely for progression of syringomyelia or the development of symptoms. © 2013 Springer-Verlag Berlin Heidelberg.