Background: Many in vitro studies have used dentine permeability to evaluate the efficacy of various restorative and preventative procedures. The easiest way to evaluate dentine permeability is to calculate its hydraulic conductance (Lp) using fluid filtration methods. Research has examined electronic hydraulic conductance measurement methods that give more precise and reliable results for the permeability of dentine than the classical method. To our knowledge, no study has examined the dentine permeability of carious primary teeth. Aim: This in vitro study determined the dentine permeability of carious primary molars precisely with a new electronic hydraulic conductance measurement system and compared them with healthy primary molars. Design: The study examined 60 carious and noncarious primary second molars with no restorations, fractures, or cracks at different root resorption stages. Results: The results showed that the dentine permeability increased with the severity of caries in primary teeth. Conclusion: The dentine permeability of carious primary teeth was higher than that of noncarious primary teeth. Therefore, treatment of primary teeth should be performed more quickly.