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Over the past few decades, the snail trail phenomenon has been known as a discoloration defect on photovoltaic (PV) modules. More recently, snail trails have also been correlated with cell microcracks in PV modules. Despite a serious concern about effects of this phenomenon on PV module performances, very few publications have dealt with this subject. The main purpose of this study is to compare the performances of polycrystalline silicon PV modules affected by the snail trail phenomenon with the reference module. The comparison was performed focusing on I-V and P-V characteristics, thermography analysis, and energy production. Measurements were carried out at the SolarTech Lab of Politecnico di Milano, Italy. The obtained results show that the snail trails may affect the polycrystalline silicon PV modules performances: Compared with reference PV, the maximum power production at standard conditions is reduced by about 40%. The energy production measured over 30 days was about 25% lower than expected.