OBJECTIVE: The somatotropic axis (growth hormone [GH] and insulinlike growth factor I [IGFI]) play a role in the cognitive deficits seen with aging, GH deficiency, and neurodegenerative disorders such as Alzheimer disease. We recently reported elevations in basal plasma GH and IGFI levels in patients with Huntington disease (HD). Here, our objective was to determine whether somatotropic axis abnormalities predicted cognitive dysfunction in HD. METHODS: In this prospective cohort study of 109 patients with genetically documented HD, aged 21 to 85 years, we determined fasting blood levels of total IGFI, GH, and insulinlike factor binding protein 3 at baseline, and we used the cognitive Unified Huntington's Disease Rating Scale to assess cognitive impairment at baseline and for up to 5 years subsequently. Associations were evaluated using mixed linear model analysis. RESULTS: Higher plasma IGFI concentrations were associated with greater cognitive decline (beta Stroop Words, -6.01, p = 0.003; beta Stroop Color, -4.41, p = 0.01; beta Stroop Color/Words, -3.86, p = 0.02; beta Symbol Digit Modalities, -3.69, p = 0.03; and beta verbal fluency, -5.01, p = 0.03). Higher free IGFI concentrations and higher GH concentrations in men also predicted greater cognitive decline. CONCLUSIONS: Our findings in patients with HD suggest that a high IGFI level at baseline may be associated with greater subsequent declines in executive function and attention.