District residents who enroll in a program to be announced Thursday will be able to instantly supply emergency responders with information that includes preexisting medical conditions, drug allergies and floor plans of their homes when they call 911, authorities said.

The program is available at no cost to residents, and registration is done online. Those who sign up will help police, firefighters and other emergency workers better understand what might await them at the scene of an emergency, according to city officials.

“When people dial 911, more often than not, something untoward has occurred,” said D.C. Fire and Emergency Services Chief Kenneth B. Ellerbe. “Those are times when panic and chaos reign, so to speak, and people forget information.”

As a result, officials say, 911 callers do not always provide potentially helpful details, especially if they are using mobile phones. A call placed from a land line will generally cause a name and an address, at least, to appear on the dispatcher’s computer screen, but cellphones are not linked to a particular address.

About 60 percent of the District’s 911 calls are placed with mobile phones, according to Jennifer Greene, director of the Office of Unified Communications, which handles the calls. Cellphone calls may yield a name and coordinates but little else, Greene said.

The new system, called Smart911, is to be unveiled by city officials Thursday, and the service should be available immediately.

Smart911 enables people to provide a range of information in advance using an online form. When someone calls 911, information stored in a profile — such as photographs, the placement of bedrooms in a house, the presence of seniors or people with disabilities, or the locations of gas valves — is displayed on the dispatcher’s computer screen.

Users can add notations to their profiles to supplement answers to standard questions. “Any information that we can get related to a home [or] individuals in a home is a plus for us,” Ellerbe said.

Smart911 will cost the District about $70,000 annually, said Tom Axbey, chief executive of Rave Mobile Safety of Framingham, Mass., which built the online form and supplies the software dispatchers use to view the profiles.

More than 300 communities across the country use Smart911, Axbey said. Once someone registers, regardless of where, the information can be seen if they call 911 in any jurisdiction using the system. Every six months, users are are asked automatically whether they want to update their profiles, Axbey said.

Persuading people to register for Smart911 has been a challenge in some communities, according to interviews with officials in several places that use the system.

Bruce Sanschargrin, an official with Nashville’s emergency communications center, could not provide the number of registrations since his city launched the program in August 2010. But he said adoption has been slow despite a television campaign and announcements made at job fairs and senior homes.

“We’re doing what we can to get folks signed up,” Sanschargrin said. “It’s just been a really slow process to get them to step up and do it.”

Gary Gray, an emergency services official in North Little Rock, Ark., said his city started using Smart911 on June 4. The city receives about 600 911 calls a day, he said, and about 10 have been from Smart911 users since the launch.

A Rave Mobile Safety spokesman said the company expects registrations to increase gradually, reaching as much as 20 percent of a jurisdiction’s population after two years.

Sanschargrin said some people may be wary about sharing too much personal information.“That’s the downside to having all this great technology,” he said. “It makes people gun-shy.”

Information entered into the system is available to no one except emergency dispatchers — and then, only when a 911 call is made, Axbey said.

Greene agreed that persuading people to register will be a challenge. Ellerbe said that in an effort to make the option available to people who don’t have easy Internet access, the city will send a WiFi-enabled van to some communities and will encourage residents to enroll at public libraries and at schools.

“It may take a while, but we have to start somewhere,” Ellerbe said.