Inclusion Criteria:

1. Males or females aged 18 years or older
2. Type 1 Gaucher disease, diagnosed by glucocerebrosidase assay or molecular analysis of the glucocerebrosidase gene.
3. Treatment with ERT for at least 3 years, with a stable dose regimen for at least the last 6 months.

4. Clinically and biologically stable disease for the previous 2 years, with at least 2 time points assessments (including Baseline as one potential time point), defined as:

   • Stable organomegaly (assessed by MRI or CT):

     • Liver volume within 10% of the mean.
     • Spleen volume within 10% of the mean.
   • Free of progressive symptomatic documented bone disease (see Appendix 1).
   • Hemoglobin levels > 11g/dl
   • Mean platelet count > 100x109 /l.

   • Chitotriosidase activity within 20% of the mean.

     • If chitotriosidase is not available (in the case of chitotriosidase deficiency, or if it was not determined), other relevant biomarkers (e.g., ACE, TRAP and ferritin) could be considered.
5. Written informed consent.

Exclusion Criteria:

1. History or evidence of oculomotor gaze palsy, ataxia or other clinical manifestations typically associated with neuronopathic type 3 Gaucher disease.
2. Not ambulant patients, or with progressive symptomatic documented bone disease (see Appendix 1).
3. Splenectomy before 18 years of age for splenomegaly and/or thrombocytopenia.
4. Peripheral polyneuropathy (not mononeuropathy) documented with both clinical signs and symptoms, and electrodiagnostic (EDX).
5. Patients (males and females) who do not agree to use reliable contraception throughout the study and for 3 months after cessation of miglustat treatment.
6. Female patients who are pregnant or breast feeding, or without pregnancy test prior to Day 1.
7. History of significant lactose intolerance.
8. Clinically significant diarrhea (>3 liquid stools per day for >7 days) without definable cause within 6 months prior to Day 1, or a history of clinically relevant gastrointestinal disorders.
9. History of cataracts or known increased risk of cataract formation.
10. Severe renal impairment i.e., with a creatinine clearance <30 ml/min/1.73mP2P
11. Concomitant active medical condition such as HIV or hepatitis B/C that would render patients unsuitable for study.
12. Previous treatment with miglustat.