MARLBORO— When a hoarder is discovered by inspectors, a neighbor or a family member, what’s found is often shocking: Items piled from floor to ceiling. Doors blocked. Stoves, refrigerators, bathtubs and sinks rendered unusable because they are obstructed or covered by piles of stuff.

In the homes of animal hoarders, the smell of urine, feces and even decaying animals can be overpowering.

Hoarding, defined as the accumulation of clutter that impedes entrances and exits to a building, or the collecting of a large number of animals in squalid conditions, has long been the bane of landlords and building inspectors. It is the second most common reason for eviction from public housing, after substance abuse, according to Thomas R. Gleason, executive director of the Massachusetts Housing Finance Agency.

Addressing a hoarding situation can be difficult, since it usually involves equal parts code enforcement and mental illness, according to Gail S. Steketee, a Boston University professor.

“The problem of hoarding has been one of the most fascinating we’ve dealt with,” she said yesterday. She was the keynote speaker yesterday at “Thinking Outside Our Box(es),” a conference at the Best Western Royal Plaza in Marlboro organized by MassHousing, a quasi-state housing finance agency. The conference drew more than 500 housing officials, inspectors, nonprofit workers and lawyers together to discuss techniques for clearing out the clutter and helping hoarders learn to stop.

Ms. Steketee told the story of an elderly gentleman who lived near her home. His entire house was filled with all manner of things. One night, the house caught fire, and he died as firefighters tried in vain to reach him. During the fire, the rear walls of his house gave way and his myriad possessions spilled out into his backyard as if poured out of a box.

In November, officials in Mendon found a farm with six hundred malnourished ducks and geese.

Ms. Steketee said her research has found that nine in 10 hoarders have some sort of mental illness, such as depression (more than 50 percent), social phobia (29 percent), generalized anxiety disorder (28 percent), and obsessive compulsive disorder (17 percent). Many hoarders show a combination of those disorders, as well as others. Dementia, particularly in the elderly, can play a factor as well.

She said that hoarders are either compulsive buyers, collectors of free things or kleptomaniacs. They save things because they remind them of someone or something important. They believe the thing might be useful someday. Or they just like to look at their things. Some hoarders don’t use their closets or cabinets because they want all of their things in plain view.

It is best to persuade a hoarder, over several visits, to begin discarding things, Ms. Steketee said. But in some cases, an inspector or health official can deem the clutter a health hazard. Officials can condemn the property or take the hoarder to court.

“How much stuff is too much?” asked Dion Irish, assistant commissioner for inspectional services for the city of Boston. “We look for health and safety issues. Are the doors blocked? Is there a fire hazard? Are there rats? Are there bedbugs?”

The cleanup can be expensive.

“We had a case where it cost us $16,000 for one cleanup,” said Mr. Gleason. After a year, the woman had once again filled her apartment to bursting, requiring a second cleanup.

Part of the problem of hoarding stems from the deinstitutionalization of the mentally ill, said Frank McCarter, tenancy preservation coordinator for Community Healthlink in Worcester.

“Now these folks are just out there, and it falls on the backs of housing complexes,” he said. “They’ve become sort of the new state hospitals, with much less expertise and much fewer resources. Institution should not be a dirty word. There’s a need for institutions.”

Ms. Steketee said that many hoarders can be taught how to stop putting themselves in danger by surrounding themselves with too many things.

“If you talk with them, they gradually talk themselves into change,” she said. “Once they have established the rules for what they will get rid of, the stuff can be moved out.”