Despite stable background pain, most cancer patients suffer 3-4 episodes of breakthrough pain daily. Aim of the present study was the evaluation of potential correlation between effective doses of sublingual fentanyl citrate, administered for controlling breakthrough pain, with transdermal fentanyl used for background pain. Fifty-six cancer patients were prospectivelly recruited. All patients were suffering episodes of breakthrough pain and managed with transdermal fentanyl 25-300μg/h for their background pain. Patients were assigned into two groups, group A (n=26) and group B (n=30), based on their transdermal fentanyl needs, 25-100μg/h or 125-300μg/h respectively. Sublingual fentanyl citrate titration was followed standard protocol, with a starting dose of 100μg. Dose effectiveness was evaluated by the patient, fifteen minutes later. If starting dose was considered inadequate, an extra dose of 100 μg was administered. The subsequent episode was treated with a starting dose of 200μg and an extra dose of 100μg, if needed, following the same stepwise approach to a maximum of 800 μg, per episode. Doses of transdermal fentanyl and sublingual fentanyl, as well as proportion of patients, titrated to 100, 200, 300, 400, 600 and 800μg of sublingual fentanyl were recorded for both groups. Statistical analysis was conducted using χ2 and Mann Whitney U tests. Correlation between transdermal fentanyl and sublingual fentanyl doses was studied using χ2 and Spearman rank correlation coefficient r. Level of significance was set at p=0.05. Transdermal fentanyl doses (mean±SD) were 65.38±27.45 and 186.66 ±62.53 μg/h for groups A and B respectively. Sublingual fentanyl doses (mean±SD) were 265.38±26 μg and 396.67±30 μg for groups A and B, revealing significant differences between groups. Proportion of patients titrated to 100, 200, 300, 400, 600 and 800 μg between groups revealed significant association between transdermal fentanyl and sublingual fentanyl. The present study showed that, despite distinguishable characteristics of breakthrough pain, compared to background pain, patients administered high doses of slow releasing opioid regimen for background pain management, required proportionally high doses of rapid onset opioid for their breakthrough pain relief.