Background: Breakthrough cancer pain (BTcP) is one of the most common symptoms related with the disease and its treatment. Its management differs from that of chronic pain but there is not a wide consensus about fundamental aspects of BTcP definition, diagnosis, follow-up, and management. The objective of this study promoted by ECO Foundation, is to reach a consensus on the follow-up of patients with BTcP under opioid treatment. Methods: This work was conducted using a modified Delphi method organized in two rounds and involving a panel of 84 medical oncologists. A questionnaire of 66 items was developed. The items proposed to reach a consensus included the following topics: 1) When to perform the first and subsequent BTcP treatment evaluations; 2) What to assess at the first follow-up visit and how; and 3) What to assess in the following visits and how. Results: After two rounds, a consensus was reached on 80.3% of the items proposed. The main agreements reached include: The first follow-up visit should preferably be face-to-face; increase the number of visits after detecting poor control of BTcP; assess pain control episodes of BTcP in the first and following visits; adjust treatment dose of opioids due to poor control of BTcP; and adopt measures to prevent aberrant opioid-induced behaviors when treating BTcP in patients with cancer. Conclusions: This Delphi consensus highlights the different points of view of medical oncologists regarding the follow-up of patients with BTcP under opioids treatment. Nonetheless, the conclusions reached can facilitate optimizing monitoring of these patients and promote longterm effectiveness of BTcP control.