OBJECTIVE Insulin-like growth factor 1 (IGF-1) provides protection against loss of dopaminergic neurons in animal models of Parkinson's disease (PD). The aim of this study was to measure serum IGF-1 in patients with PD and assess its correlation with the clinical presentation. METHODS Serum IGF-1 and growth hormone (GH) levels were measured in 12 patients with idiopathic PD and 12 matched healthy controls, three times over a period of 6 months after overnight fasting. Based on the results, serum IGF-1 was measured in six additional, yet untreated, PD patients. RESULTS IGF-1 values were stable over the whole period (r=0.83-0.93) in patients and controls. IGF-1 was significantly higher in treated PD patients than in controls at all time points (all p<0.001). There were no significant differences in serum GH levels between patients and controls. Receiver operating characteristics showed a cut-off at 114 ng/ml for differentiation of treated patients and controls (area under the curve=0.90). In the patient group, higher serum IGF-1 levels were correlated with shorter disease duration (r=-0.56, p=0.001). In the healthy control group, higher IGF-1 was correlated with slightly impaired motor performance, as measured by the Unified Parkinson's Disease Rating Scale motor score (r=0.46, p=0.005). In the untreated patient group, serum IGF-1 levels were significantly higher than in healthy controls (p<0.001). Applying the cut-off value of 114 ng/ml, all untreated patients were correctly classified as PD patients. CONCLUSIONS Increased IGF-1 might be an interesting serum marker for early PD and potentially for subclinical dopaminergic dysfunction.