How many cartels are there, and how long do they live? The answers to these questions are important in assessing the need for competition policy. We present a Hidden Markov Model that takes into account that often it is not known whether a cartel exists or not. We take the model to data from a period of legal cartels-Finnish manufacturing industries 1951-1990. Our estimates suggest that once born, cartels are persistent; by the end of the period, almost all industries were cartelized.