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To discuss recent reports on the function and importance of the renal primary cilium, a widely distributed organelle.Most epithelial cells, including those in the kidney, express a solitary primary cilium. The primary cilium functions as a flow sensor in cultured renal epithelial cells (MDCK and mouse collecting tubule) mediating a large increase in intracellular calcium concentration. Flow sensing is shown to reside in the cilium itself and to involve the proteins polycystin 1 and 2, defects in which are associated with the majority of cases of human polycystic kidney disease. The role of the cilium in flow-dependent potassium secretion by the collecting tubule and in sensing of chemical components of the luminal fluid are also described.The primary cilium is mechanically sensitive and serves as a flow sensor in cultured renal epithelia. Bending the cilium by mechanical means or flow causes a large, prolonged transient increase in intracellular calcium. The mechanically sensitive protein in the cilium is a polycystin.