Several theoretical propositions suggest that changes in economic activity can explain the decision to form a cartel. The majority of the propositions claim that cartel formation is more likely in expansionary and less likely in contractionary phases of the business cycle. The propositions are re-examined theoretically and by using data on detected cartels for the European Union. In both cases, the results cannot confirm that more cartels are formed in any of the business cycle phases and that correlated economic growth rates create higher incentives to collude. Furthermore, it cannot be confirmed that more cartels form shortly after respectively the trough or the peak in the business cycle. The conclusion is that cartel formation is unaffected by changes in the business cycle.