Thin-airfoil theory is applied to steady, plane potential flow about vented or cavitating hydrofoils of arbitrary profile when there are two free-streamlines detaching from the foil and bounding the single cavity that extends downstream of the trailing edge. Cavity-termination models employed are the closed, the partly closed and the open models for which the thickness of the implied ’wake’ following the cavity ranges from zero to maximum for the open model. The general solution for given wetted-surface profile, cavity length and particular cavity termination is constructed by superposition of the profile's cusp-closure solution (angle of attack α+) plus the particular flat-plate solution to give the desired angle of attack α. Four related integrals involving the wetted-surface contour slope distribution lead to drag, lift, cavity pressure and α+vs cavity length. A comparison of theoretical and experimental lift and drag for a cavitating hydrofoil shows good agreement until the theoretical cavity closure nears the trailing edge.