The deadly pacts pose a grim challenge for officials struggling to deal with Japan's high suicide rate, one of the worst among industrialized nations.
No religious prohibitions exist against taking one's own life in Japan, where suicide was once a form of ritual atonement for samurai warriors and in modern times is a way to escape failure or save loved ones from embarrassment or financial loss.
Suicides surged by 35 percent in 1998 as Japan's economy was mired in stagnation and have exceeded 30,000 every year since then.
Group suicides make up only a small fraction of the total, but the steady annual increase, along with the widespread media coverage that most receive, has experts increasingly worried.
"Many people are too scared to die alone," said Yumiko Misaki, director of the Tokyo Inochi no Denwa (Phone of Life), a suicide counseling service. "So they reach each other through the Internet and make arrangements?And the worst thing is that people are often very influenced by reporting on this, so it's likely to keep on increasing."
In 2003, 34 Japanese died in group suicides. That rose to 55 in 2004 and then 91 last year.
There were a total of 32,325 suicides in Japan in 2004, the latest year for which figures are available. That is down from the record high of 34,427 in 2003 but second only to Russia among Group of Eight industrialized nations.
According to World Health Organization data, Japan's suicide rate was 24.1 per 100,000 people in 2000, compared with 39.4 in Russia and 10.4 in the United States.
The pace of group suicides was especially sharp during the first three months of last year. On one day in February, six people were found dead in a car on a deserted rural road.
As in most of the other cases, police found several charcoal stoves in the car, which had its windows sealed from the inside. The three men and three women had died by inhaling carbon monoxide from the charcoal.
Experts warn that the Internet alone cannot be blamed for promoting suicide, but noted that the intensity of some suicide chat rooms may worsen the psychological state of those involved.
With mental care systems in Japan still basic and often overloaded, the Internet also has the potential to be a powerful therapeutic tool, particularly since many Japanese find it hard to share their worries with others face to face.
The time lag between people writing about their feelings and receiving an answer, however, is a hurdle that Misaki's group--which plans to start an Internet counseling service later this year--finds worrying.
"The best solution would be if we could break into the chat rooms and start communicating with people directly," Misaki said.
There are some hopeful signs, however.
From October, several communications industry groups began providing police with information on people who posted messages suggesting they might be close to committing suicide.
Deaths from group suicides in the last three months of the year, after the new system took effect, dropped to 11, down from 36 during the same period the previous year, police said.