Fracture probabilities associated with spherical cavities have been analyzed, by combining principles of fracture mechanics and fracture statistics. The analysis considers that fracture occurs from a distribution of cracks located at the cavity surfaces. It predicts effects of cavity size and cavity volume content (porosity) on strength, that depend sensitively on the flaw population vis-a-vis the cavity size distribution. The theory is shown to have limit solutions that coincide with several earlier models of fracture, derived for porous ceramics. The predicted effects of pore size on strength are compared with some available data.