We thank Dr Manfredini and colleagues for their interest in our work.1 We agree that from the analysis of the administrative data, we are unable to determine a cause-and-effect relationship for the weekend effect. As mentioned in their letter, we did find that fewer people than expected with pulmonary embolism (PE) were admitted over the weekend than during the week. Weekend admissions for PE also had higher severity of illness as arbitrated by the need for mechanical ventilation, thrombolysis, and vasopressors. However, the weekend effect persisted when we controlled for all these factors, demographics, and random hospital effects, suggesting that perhaps other variables than those mentioned are responsible. We are also in agreement that carefully planned studies should be devoted to investigate the weekend effect in PE and other acute medical conditions.