A positive crankcase ventilation system for an internal combustion engine is disclosed. The system includes an engine having a cylinder air intake system connected to associated cylinders and a filtered air inlet to a crankcase for admitting air to mix with crankcase vapors. A throttle is disposed in the cylinder air intake system for controlling airflow to the associated cylinders. The system further includes a venturi nozzle having an inlet and an outlet. The venturi nozzle inlet is connected to the crankcase for receiving the mixture of filtered air and crankcase vapors. The venturi nozzle outlet is connected to the cylinder air intake system at a location subject to variable intake vacuum pressure between the throttle and the cylinders to allow the mixture of filtered air and crankcase vapors to be drawn into the inlet air passing to the cylinders downstream of the throttle.