Much pre-clinical evidence show that insulin-like growth factor 1 (IGF-1) provides protection against loss of dopaminergic neurons. Recently, IGF-1 has been proposed as a possible biomarker for early diagnosis of Parkinson's disease (PD). We aimed to assess the relationship between serum IGF-1 levels and progression of motor symptoms in a cohort of drug-naïve PD patients. Serum IGF-1 was measured at baseline in 37 early, drug-naive PD patients; subsequently, patients were evaluated "on drug" by means of UPDRS-III, UPDRS dopa-resistant score and dopaminergic score at 12, 18 and 24 month follow-up. Repeated measures ANOVA was used both to evaluate progression of motor scores within time and differences between serum IGF-1 quartiles, age at onset and motor phenotype. Patients at the highest IGF-1 quartile were found to have significantly higher UPDRS-III (p < 0.001) and dopaminergic score (p < 0.001), as compared to patients at other quartiles. Mean serum IGF-1 level was moderately increased in PD as compared to healthy controls (p < 0.011). IGF-1 levels are related to those symptoms predominantly responsive to dopaminergic treatment. This is the first study to demonstrate a relationship between serum IGF-1 and progression of motor symptoms in the early stage of disease.