This twin register was organized as a resource for studies on the eye. It has been possible to share this resource with other disciplines through effective collaboration. Conversely, only through the assistance of other investigators who are working with twins, will it be practical to undertake certain studies which are important in ophthalmology. The purpose of this presentation is to convey a description of our register and some of its limitations, and to specify the need for collaboration with others.

The original purpose in assembling this register was to identify a group of twins readily available for multiple clinical examinations. Since the first examining facility was to be located near the National Institutes of Health, practical considerations required that twin residence be limited to the metropolitan Washington, D. C. area. Twins of all ages were included in the register, since proposed investigations pertained to different age categories. No effort was made to select twins through randomization procedures; all twins who were identified were invited to membership in the register. A variety of methods were used to locate twins, including contact with schools and clubs, but the largest proportion of registrants were recommended by other twins.

Twins were offered a thorough eye examination, along with the solicitation of their interest in participating in future research activities. The clinical examination was offered for the following reasons:

1) To encourage informed interest in proposed future investigations by acquainting the twins with our facility and purpose;

2) To perform base-line observations, so as to identify and enumerate ocular characteristics as an aid in selecting subsets of twins for future studies;

3) To obtain data for possible estimates of heritability of selected ocular characteristics.