Several hundred twins from across Britain and a few from abroad attended a garden party at St Thomas' Hospital on Saturday.
The Twin Summer Party was hosted by King's College London's Department of Twin Research. Big Ben across the river provided a backdrop for photographs.
"We are thanking twins for helping us with research," said Professor Tim Spector of the Twins Research Unit and Twins UK. "They do so voluntarily."
There are 11,000 twins on the database and Prof Spector is anxious to tell the public how important twin research has been in helping science. During the day the guests took part in life experiments involving puzzles and other fun tests at various stalls.
"We want to promote more twins coming forward, volunteering and helping us," said the professor. "Twin research has helped science by telling us which diseases are strongly genetically influenced so we focus us our research on them."
The department, which has a staff of 45, has published more than 400 research papers based on data collected from the twins.