The California Twin Program (CTP) is a population-based sample of over 52,000 twins in which a number of nested studies are ongoing. We outline our experience to date, providing estimates of crude response rates for a variety of different study designs and protocols. We have experienced very high response rates in our studies to date, even in studies with demanding protocols. Lowest response rates have occurred in studies among afflicted individuals, and in one with an unusual protocol. We have experienced some difficulty in locating original members of the cohort, despite efforts to trace individuals using a variety of sources of information. However, in most analyses, the participating sample of twins does not differ substantially from the underlying sample from the CTP. Future work will focus on improving methods of re-contacting cohort members.