A leak resistant drinking cup has a container for holding liquid and a lid assembly selectively attachable to the container. A diaphragm of the cup includes a liquid discharge member and a sealing member adapted for movement between a sealed position in which the sealing member blocks the flow of liquid from the container to the liquid discharge member, and an unsealed position in which liquid is permitted to flow from the container to the liquid discharge member. The liquid discharge member and sealing member are formed as one-piece.