This investigation demonstrates the reliability of fluorescein for detecting the permeability of incipient dental caries (white spots). Artificial white spots were created on the buccal surface of 12 human bicuspids by viscous lactic acid (pH 4). Permeability of these lesions was assessed and reassessed before and after 24 and 48 hr of acid challenge using two disclosants: sodium iodide and sodium fluorescein. Estimates obtained from both disclosants showed that the microvoid volume approximately doubled as the decalcification time doubled. The two disclosants exhibited good intraclass reliability and their scores were correlated (r = 0.69 to r = 0.91). However, only fluorescein disclosed the extent of porous white spot lesions. Thus, fluorescein should be considered when the objective is to detect the location and permeability of incipient lesions.