Melanoma is a widespread cancer with poor prognosis. Female hormones are known to be capable of influencing melanoma progression but clinical data related to pregnancy, oral contraception and hormone replacement therapy are controversial. A few reports show that in vitro progesterone (PG) affects melanoma growth in nuclear progesterone receptor (nPR)-positive and nPR-negative cells, but the experimental protocols used are quite different and the results are not univocal. Further research on this topic is thus needed especially in view of the widespread use of PG in clinical practice. In this study, we used human melanoma cells (A-375), which were cultured in vitro in the presence or absence of a wide range of PG concentrations (from 0.01 to 1000 μM) in single treatment. Daily cell count, cell cycle analysis and apoptosis assay were performed. Our results show that the low PG concentrations (from 0.01 to 1.0 μM) promote a significant increase of melanoma cell proliferation but this growth-stimulatory effect is not observed at 10 μM PG and the higher PG concentrations (i.e. 100 and 1000 μM ) induce a cell density reduction which is the result of both cell cycle arrest and apoptosis. These findings confirm and extend previous observations reported in the international literature. A higher caution in the clinical use of progesterone is thus mandatory, since PG concentrations capable of stimulating melanoma cell proliferation are very close to those commonly used in a wide spectrum of physio-pathological conditions.