Abstract: A pilot-operated diaphragm valve in which the diaphragm seats on a valve seat having an unconstricted flow annulus therearound, and support regions are provided on the valve seat and on the valve body for support of the diaphragm when in closed position. The support region on the valve seat further performs important sealing functions, as does a sealing lip disposed radially-inwardly of the sealing and support region of the valve seat. To stabilize the diaphragm at various partially and fully open positions, and to augment sealing action when the valve is closed, a spring-pressed poppet is disposed above the diaphragm and is guided by a valve stem and bearing. A peripheral region of the poppet bears a certain relationship to the sealing portions of the valve seat.