The effect of pressure on porosity in cast A356.2 was investigated. Alloys were melted in an induction furnace under vacuum and then pressurized with argon and 0.2 atm hydrogen to a total overpressure of either 1 atm, 10 atm, or 20 atm. Hydrogen was intentionally added to effect porosity, so that the role of overpressure on suppressing the porosity could be examined. While maintaining an overpressure, the alloys were poured into bottom-chilled molds, resulting in castings that were in turn sectioned, polished, and analyzed for porosity. The resulting data show that an increase in overpressure reduces the amount of porosity and shifts the pore size distribution to smaller pores. These findings were further supported by modeling the casting process with a finite element model of the formation of porosity during solidification using the same alloy and experimental conditions. By solving the redistribution and transport phenomena of hydrogen during solidification and comparing its Sievert's pressure to the local pressure within the alloy cast under pressure, it was found that the predicted results agreed reasonably well with the experimental data.