Obstructive sleep apnoea (OSA) is reported to have effects on a number of hormone systems including the hypothalamo-pituitary axis. We aimed to determine the impact of OSA severity on insulin-like growth factor-I (IGF-I) levels. This is a prospective cohort study performed between November 2014 and May 2017. IGF-I was measured on serum samples, and data were collected on demographics, BMI and parameters of OSA. 611 participants were recruited (202 female, 53.5±12.5years; mean BMI 36.2±8.0kg/m2 ). 26.2% had mild OSA; 27.3%, moderate OSA; and 44.5%, severe OSA. 15.2% of IGF-I values were below the age-related reference range. Increasing BMI correlated with greater AHI (r=.28, p<.001), ODI (r=.30, p<.001), severity of OSA (r=.17, p<.001), duration with oxygen saturation (SaO2 ) <90% (r=.29, p=.001) and reduced median SaO2 levels (r=.19, p<.001). IGF-I levels correlated negatively with age (r=-.13, p=.001), BMI (r=-.16, p<.001), diabetes (r=-.108, p=.009), AHI (r=-0.10, p=.043) and severity of OSA (r=-.10, p=.013). No association of IGF-I was observed with ODI, median SaO2 levels or duration of SaO2 <90%. Regression analyses were used to examine determinants of IGF-I, all of which contained the independent variables of age, gender and BMI. All models showed IGF-I to be predicted by age and BMI (p<.05); however, none of the parameters of OSA were significant within these models. Insulin-like growth factor-I levels in OSA are dependent on age and BMI; however, no additional effect of any OSA parameter was observed, supporting the hypothesis that OSA effects on IGF-I are indirect through concomitant body composition and metabolic parameters.