The purpose of this clinical trial is to extend the Pre-Crest study (Protocol # (NCT00592995) to further assess the long-term safety and tolerability of up to 30 grams daily creatine in individuals at-risk for Huntington's Disease (HD) and to assess whether biomarkers responsive to creatine in symptomatic individuals are informative in premanifest individuals over a longer duration.

Extensive evidence exists that neurodegeneration begins many years before HD can be diagnosed clinically. Therefore, it is most desirable to begin a neuroprotective therapy before or during this premanifest period with the aim of delaying onset, as well as slowing functional decline. Cellular energy depletion is present early in HD and can be ameliorated by creatine, which helps regenerate cellular ATP. Preclinical evidence for creatine's potential neuroprotective effects in animal models of HD has been well-documented. Before the clinical efficacy of creatine can be tested in premanifest HD, its long-term safety and tolerability must be assessed in these individuals and its ability to favorably modify biomarkers of HD should also be confirmed. This extension trial will continue to follow eligible individuals who completed the PreCREST study on open-label creatine (up to 30 grams daily) for long term safety and tolerability. Additional biological and imaging biomarkers of disease progression and potential response to treatment will also be assessed.

Inclusion Criteria:

• Individuals who have completed the Pre-CREST Study.
• Individuals capable of providing independent informed consent and complying with trial procedures.

Exclusion Criteria:

-Clinical evidence of unstable medical or psychiatric illness in the investigator's judgment.

Additional eligibility criteria apply.