Abstract Aims It is unknown whether cardioversion of atrial fibrillation causes thromboembolic events or is a risk marker. To assess causality, we examined the temporal pattern of thromboembolism in patients having cardioversion. Methods and results We studied patients randomized to aspirin or aspirin plus clopidogrel in the ACTIVE trials, comparing the thromboembolic rate in the peri-cardioversion period (30 days before until 30 days after) to the rate during follow-up, remote from cardioversion. Among 962 patients, the 30-day thromboembolic rate remote from cardioversion was 0.16%; while it was 0.73% in the peri-cardioversion period [hazard ratio (HR) 4.1, 95% confidence interval (CI) 2.1–7.9]. The 30-day thromboembolic rates in the periods immediately before and after cardioversion were 0.47% and 0.96%, respectively (HR 2.2, 95% CI 0.7–7.1). Heart failure (HF) hospitalization increased in the peri-cardioversion period (HR 11.5, 95% CI 6.8–19.4). Compared to baseline, the thromboembolic rate in the 30 days following cardioversion was increased both in patients who received oral anticoagulation or a transoesophageal echocardiogram prior to cardioversion (HR 7.9, 95% CI 2.8–22.4) and in those who did not (HR 4.8, 95% CI 1.6–14.9) (interaction P = 0.2); the risk was also increased with successful (HR 4.5; 95% CI 2.0–10.5) and unsuccessful (HR 10.2; 95% CI 2.3–44.9) cardioversion. Conclusions Thromboembolic risk increased in the 30 days before cardioversion and persisted until 30 days post-cardioversion, in a pattern similar to HF hospitalization. These data suggest that the increased thromboembolic risk around the time of cardioversion may not be entirely causal, but confounded by the overall clinical deterioration of patients requiring cardioversion.