A potential mechanism for liquid drainage in wet porous media based on cavitation of water (the spontaneous formation of water vapor bubbles) under tension is proposed. Cavitation theory applied to an idealized pore predicts considerable differences in drainage branches of characteristic curves determined under tension and with imposed positive pressure. A potentially important aspect of cavitation is the possibility of liquid displacement (drainage) even in the absence of a continuous gaseous phase. Surface heterogeneity and impurities in soil and rock water are conducive for lowering cavitation tension threshold; on the other hand, small pore sizes and viscous resistance to flow restrain cavitation in natural porous media. The significance of cavitation in flow and transport processes is yet unknown; however, there are two important aspects to this phenomenon: (1) it may require revision of several conceptual models for liquid retention and displacement in porous media, and (2) extrapolating from evidence in plant xylem systems, cavitation may play an important role in determining the unsaturated hydraulic conductivity function.