The Swedish Twin Registry in the third millennium: an update.

The Swedish Twin Registry was first established in the late 1950s. Today it includes more than 170,000 twins--in principle, all twins born in Sweden since 1886. In this article we describe some ongoing and recently completed projects based on the registry. In particular, we describe recent efforts to screen all twins born between 1959 and 1985, and young twin pairs when they turn 9 and 12 years of age. For these studies, we present initial frequencies of common conditions and exposures.