A third of American teenagers have attended parties where parents were at home while alcohol or illegal drugs were used, according to an annual back-to-school survey on teens’ attitudes that paints an overall portrait of a generation of parents clueless about their kids’ vices.

The study did not suggest that parents were aware of what was happening when teenagers were partying in their homes. To the contrary, only 12 percent of parents see drugs and alcohol as a problem for their children, while 27 percent of teens ranked it their biggest concern. Fifty-eight percent of parents cited social pressure as their child’s biggest issue.

“These parents are like the three monkeys,” said Joseph Califano Jr., chairman and president of the National Center on Addiction and Substance Abuse at Columbia University. “They see no beer, smell no pot and don’t hear all hell breaking loose in the family room because of the alcohol and drugs. : It doesn’t take a sharp nose to smell pot.

“These parents don’t understand the world their children are living in. There is a lot of denial here.”

The center, which studies risk factors for substance abuse, this year took a particularly close look at parents and delved into the private party scene, where much of the underage drinking and drug abuse are thought to occur. In a number of areas, the opinions of parents were strikingly at odds with the views of their children.

The study found that 80 percent of parents think that neither alcohol nor marijuana is usually available at parties that their teenagers attend. Fifty percent of teenagers said they had been at parties where alcohol and drugs were being used.

For the first time, the survey also found that the substance-abuse gender gap has closed, with girls 12 to 17 at equal or higher risk compared with boys.

Family structure also showed up as a strong indicator of substance-abuse risk. Teenagers who regularly ate dinner with their families and attended church services were at less risk, as were teenagers who slept more than eight hours a day.