Bobko et al. reported a very interesting article concerning the impact of interstitial inorganic phosphate (Pi) on tumor progression. Previous studies have shown that blood levels of Pi might be related with either the presence or growth of cancer in the human body. Heroin-addicted persons have normal values of Pi, while the incidence of cancer in these individuals seems to be very low. The question rises: Is Pi the key of cancer? Further studies are definitely needed, focusing on the correlation between inorganic Pi and cancer.