Changes in turning are one of the early motor deficiencies in Parkinson's Disease (PD). We have proposed a system based on wearable, inertial sensors and a novel automatic analysis algorithm that can assess 180° turns. Twelve patients in early stages of PD and 14 age-matched healthy subjects were enrolled in this study. Inertial sensors were attached on shanks and sternum. Measurement protocol included walking on a straight pathway, turning 180° and returning back. Subjects were measured 4 times, once every 6 months during an 18 months period. At the baseline, 9 subjects from each group repeated the test twice to assess test-retest reliability. Patients with mild PD had a very low Postural Instability Gait Difficulty (PIGD subscore of UPDRS III) score (average 0.67, min 0, max 3). The analysis showed that the patients had a significantly longer turning duration (2.18±0.43 vs. 1.79±0.27 seconds, p<0.02) and longer delay in their last step before initiating a turn (0.56±0.04 vs. 0.52±0.04 seconds, p<0.03). Estimated turning duration and other metrics had a high test-retest reliability (ρ>0.85). Turning duration also showed a significant Group*Time interaction (p<0.03) during the longitudinal study highlighting early signs of the progression of the disease.