Introduction: During the last 10 years, the centurylong steady decline in the annual numbers of Danish-born tuberculosis (TB) cases seems to have stagnated, primarily due to an increasing number of cases among young to middle-aged Danish-born men who are part of the »Danish Cluster 2« outbreak. Materials and methods: Since 1992, DNA analysis of M. tuberculosis strains has been implemented in Denmark on a nationwide basis, using the internationally standardized restriction fragment length polymorphism method. This study included 4,102 DNA fingerprints representing 97% of all culture-positive TB patients diagnosed in Denmark from 1992 through 2001. Results: One specific strain emerged rapidly. In 1992, the Danish Cluster 2 strain accounted for 6% of all culture-positive Danish-born cases, increasing to 29% in 2001. The patients infected with Cluster 2 strains were on average younger, more likely to be male and more likely to have pulmonary involvement than other Danish-born cases. During the first four observation years, they were found mainly in the capital city (Copenhagen) but later on increasingly in the provinces. Discussion: We consider the increasing dominance of Cluster 2 cases and the change in their geographical distribution as an outbreak. The situation requires an increased focus on early diagnosis of tuberculosis and control of its transmission in Denmark.