Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in genes encoding the polycystin (PC) 1 and 2 proteins. The goal of this study was to determine the role of calcium in regulating cyst growth. Stromal interaction molecule 1 (STIM1) protein expression was 15-fold higher in PC1-null proximal tubule cells (PN) than in heterozygote (PH) controls and 2-fold higher in an inducible, PC1 knockout, mouse model of ADPKD compared to a non-cystic match control. IP3 receptor protein expression was also higher in the cystic mice. Knocking down STIM1 with siRNA reduced cyst growth and lowered cAMP levels in PN cells. Fura2 measurements of intracellular Ca showed higher levels of intracellular Ca, SOCE and thaspigargin-stimulated ER Ca release in PN vs. PH cells. There was a dramatic reduction in thapsigargin-stimulated release of ER Ca following STIM1 silencing or application of 2-APB, consistent with altered ER Ca movement; the protein expression of the Ca-dependent adenylyl cyclases (AC) AC3 and AC6 was up- and down-regulated, respectively. Like STIM1 knockdown, application of the calmodulin inhibitor W7 lowered cAMP levels, further indicating that STIM1 regulates AC3 via Ca We conclude that the high levels of STIM1 in ADPKD cells play a role in supporting cyst growth and promoting high cAMP levels and an increased release of Ca from the ER. Thus, our results provide novel therapeutic targets for treating ADPKD.