Immigration from high prevalence areas may contribute to an increased risk of tuberculosis in Europe. This study aimed at quantifying transmission of tuberculosis between and within nationalities among residents of the Netherlands. DNA "fingerprints," on the basis of restriction fragment length polymorphism using marker IS6110, were made of all Mycobacterium tuberculosis isolates in the Netherlands from January 1993 through June 1995. Clusters were defined as groups of patients that had isolates with identical fingerprints. It was assumed that the probability of a patient being the source of a cluster was proportional to the incidence rate of potential sources times the probability that a potential source would give rise to a cluster. The transmission index was defined as the average number of secondary cases of infectious tuberculosis caused directly or indirectly through recent transmission by a single potential source case and was used to estimate the effective reproductive rate associated with recent transmission, ReFAST. Among a total of 623 Dutch tuberculosis cases, 17% (95% confidence interval 9-25%) of cases were attributable to recent transmission from a non-Dutch source. The transmission index varied strongly by nationality, and was highest among the Surinamese (1.3), Moroccan (0.8), and Turkish (0.8) populations; ReFAST was 0.26. Aggregation of tuberculosis cases of given nationalities within clusters was most pronounced among recent immigrants from Somalia and (ex-)Yugoslavia. The authors conclude that differences in transmission between subpopulations can be quantified and may be used to evaluate and direct tuberculosis control.