Background: This study determined if there are observable patient-, tooth- and crack-level characteristics markedly associated with whether a tooth with an external crack also has an internal crack. Methods: Two hundred nine dentists in The National Dental Practice-Based Research Network enrolled 2,858 adults with a vital permanent posterior tooth having at least 1 observed external crack. Presence and characteristics of internal cracks were recorded for 435 cracked teeth that were treated. Generalized estimating equations were used to identify significant (P <.05) independent odds ratios associated with the tooth having internal cracks. Results: Overall, 389 teeth (89%) had at least 1 internal crack, with 46% of these teeth having 2 or more internal cracks. Sixty-nine percent of treated cracked teeth were associated with 1 or more types of pain assessed before treatment; 53% were associated with cold testing, 37% with bite testing, and 26% with spontaneous pain. In the final model, biting pain, having an external crack that connected with a restoration, or an external crack that extended onto the root was each associated with more than a 2-fold increased odds of having an internal crack. Conclusions: Essentially 9 of 10 teeth that had at least 1 external crack also had at least 1 internal crack. Practical Implications: The external cracks that a dental practitioner should be most concerned about, because they are most likely to be associated with internal cracks in the tooth, are those in which the patient experiences biting pain, is connected with a restoration of some type, or extends onto the root.