The authors sought to validate the defining characteristics of acute pain and chronic pain and to compare the differences between them. Expert nurses (N = 125) rated the importance of 55 clinical indicators for each diagnosis (acute pain and chronic pain). Differential diagnostic validity (DDV) scores were calculated for each clinical indicator for both diagnoses. Only acute pain had a DDV score greater than .80, indicating that the characteristic "communication of pain descriptors" was a critical indicator for acute pain. A majority of defining characteristics differentiated between acute pain and chronic pain, thereby supporting the identification of acute pain and chronic pain as separate nursing diagnoses.