BACKGROUND AND OBJECTIVE: AZD1305 is an investigational antiarrhythmic agent that prolongs refractoriness through combined potassium and sodium channel inhibition. This study aimed to explore the utility of a test dose in predicting QT interval corrected according to Fridericia's formula (QTcF) during subsequent maintenance treatment with AZD1305. METHODS: This was a randomized, double-blind, parallel-group, placebo-controlled trial carried out at multiple hospital cardiac facilities in Denmark, Norway, Poland, Slovakia, and Sweden. Patients with documented atrial fibrillation (AF) but currently in stable sinus rhythm for ≥2 hours and ≤90 days were eligible for inclusion. Patients were randomized in a 1 : 1 : 1 ratio to receive AZD1305 extended-release or matching placebo tablets as follows: group A - test dose 250 mg, evening dose 125 mg on day 1, maintenance dose 125 mg twice daily; group B - test dose 500 mg, placebo evening dose, maintenance dose 125 mg twice daily; placebo group - placebo test and maintenance dose. Maintenance dosing was for 9 days. QTcF >550 ms at any time during the in-patient phase or >500 ms after discharge (day 4) were predefined study drug discontinuation criteria. The main outcome measure was the relationship between QTcF following the test dose and during maintenance treatment. RESULTS: Sixty-five patients were randomized (n = 21, 22, and 22 in group A, group B, and the placebo group, respectively). AZD1305 dose-dependently increased QTcF. There was a positive, linear correlation between the change in QTcF during the first 6 hours after the test dose and during the maintenance phase. Three patients, all from group B, discontinued treatment on day 1 due to QTcF >550 ms. All other patients completed the study without events related to QT prolongation. There was a trend for reduced AF recurrence with AZD1305 compared with placebo. CONCLUSION: In this exploratory study a test dose predicted the QT response during maintenance treatment with AZD1305 and may thus be employed in further studies. [ClinicalTrials.gov Identifier: NCT00643448].