Diagnostic uncertainty, the perception of a lack of or incorrect label to explain symptoms, has been reported by parents of youth with chronic pain. This study was the first to examine diagnostic uncertainty in both youth with chronic pain and their parents using qualitative methodology. Individual, face-to-face, semi-structured interviews were conducted with twenty youth with chronic pain recruited from a pediatric chronic pain program. Independent interviews were also conducted with one of their parents. Interviews explored participants' memories and perceptions around diagnosis. In-depth thematic analysis revealed four themes: (1) The Function of a Diagnosis. Parents and youth struggled with the meaning of the diagnosis, needed further explanation for the pain, and perceived the 'right' diagnosis (i.e., one that fit with their beliefs) as justification for the pain. (2) Haunted by Something Missing. Negative test results did not provide relief or counter the belief that something serious could have been missed by clinicians. (3) The Search for an Alternative Diagnosis. A search persisted for the 'right' diagnosis, particularly when a non-pharmacological treatment plan was provided. (4) Mistrust in the Medical System. Clinician communication and perceptions of clinicians' uncertainty impacted parent and youth 'buy in' to the diagnosis. Findings suggest that many youth with chronic pain and their parents experience diagnostic uncertainty, which is integrally tied to their past experiences with the medical system. Greater understanding of diagnostic uncertainty may help tailor how clinicians deliver diagnoses to achieve 'buy in', increase understanding of pain and diagnosis, and improve treatment response. Perspective: A major challenge that youth with chronic pain and their parents face is understanding the cause of the pain. Youth with chronic pain and their parents experience uncertainty about their diagnosis, which may be linked to their 'buy in' and treatment response.