The Sri Lankan Twin Registry began as a volunteer register in 1997. Previously, we have shown door-to-door surveys as the best option to build a population-based twin register of older, adult twins. Our aim was to build a population-based twin register in the Colombo district and use it for twin studies on common mental disorders. We aimed to identify a random sample of 4000 twins ascertained through the twin census. The Colombo district is divided into 13 divisional secretariat divisions (DSDs) and each division is further divided into Grama Niladari divisions administratively. Grama niladaris (GNs) are civil servants and visit each household in order to update the electoral register. GNs were used to do a census of twins while they updated the electoral register. The correlation between population density and twin rate among DSDs was .81, and between twin rate and the percentage of returned forms (from each DSD) was .74. We received 9648 forms notifying about twins and multiples. After removing the duplicates and information about twins who live in other districts, there were 17,406 multiples remaining. After incorporating twins and multiples from various other feasibility studies we now have 19,040 multiples with 8.46 twins per 1000 people in the Colombo district.