On a 2003 trip to Toronto with her husband, Sharon Elfenbein of Denver picked up a copy of the Toronto Star one morning and came across an insert about an event called Doors Open Toronto. Started in 2000 by the City of Toronto, the two-day event offers lovers of architecture the opportunity to explore dozens of buildings that are either closed to the public or normally charge an entrance fee. (This year’s event will be May 29 and 30.) Newspaper guide in hand, the Elfenbeins decided to spend a day visiting some of Toronto’s architectural treasures.

A shot of the building that now houses Anderson Mason Dale Architects. It was once a car dealer, then a pennant factory. The building is part of this year's Doors Open Denver, which features buildings that have a "second life."

Click on the slide show icon to see additional photos.

“We saw some very interesting buildings,” Sharon Elfenbein says. “And on the way back home, we thought, Why not do this in Denver?” Elfenbein, who had just retired from her position as an aide to a Denver city councilwoman, convinced Mayor John Hickenlooper to get on board, and in April of 2005, Doors Open Denver was launched.

Five years later, Doors Open Denver, which bills itself as an annual celebration of the city’s built environment and design, has become a popular two-day event for Denver architecture fans. On the morning of the first day, hundreds of eager participants line up outside of historic Union Station, event headquarters, to get free tickets for various walking tours, often led by architects. (Expert tours are optional; most buildings are open to anyone who wants to walk in.) Last year, even a powerful spring blizzard—which dumped more than a foot of snow on the city—failed to dampen the enthusiasm of devotees.

The theme for this year’s event, scheduled for April 17 and 18, is “Denver Redux/Denver Redo: Buildings with a Second Life.” Among the 80 buildings and sites are a car dealership turned architecture firm, a seed company turned public relations firm, and a paint warehouse turned loft space.

“It’s wonderful to show people really good architecture and design,” says Elfenbein, now chairwoman of the DOD Leadership Council, “but it’s also wonderful to show off the city and to make people proud of what is here.”

A number of cities around the world have jumped on the “Doors Open Days” bandwagon in recent years. The idea seems to have started in France in the 1980s, and it then spread to other European locations such as Glasgow and London. Lowell, Massachusetts, claims to be the first U.S. city to launch a Doors Open event, in 2002. (This year’s will be May 14-16.) Like Denver’s, it was inspired by Doors Open Toronto.

The biggest in the United States appears to be Open House New York, started in 2003 and held on one weekend every October, with other events scheduled throughout the year. Last year, according to Executive Director Renee Schacht, more than 185,000 people visited 192 sites in all five boroughs of New York. With three full-time employees and an annual budget of about $300,000, OHNY is the envy of Doors Open Denver officials, who hope to expand DOD to include year-round events.

That will take more funding, however, which isn’t easy in the current economy. For the near future, Doors Open Denver will continue as an annual two-day event. Sponsored by the Denver Architectural Foundation, its annual $100,000 budget comes from various sources, including the City of Denver and the National Endowment for the Arts. Dennis Humphries, principal of Humphries Poli Architects, leads the fundraising efforts, while DOD’s one full-time employee, Project Manager Carol Hiller, handles the logistics of selecting buildings and organizing volunteers. She also spends a fair amount of time talking to officials from other cities intrigued by the Doors Open concept.

“More and more cities want to learn how to do this,” she says, “and they come to us to see how it’s done.”