A 2-year-old boy survived an 80-foot plunge to the concrete sidewalk his apartment home at 1200 Delaware Ave. SW yesterday while his mother and two sisters were inside the unit, unaware at the time that he had fallen.

The boy, Lamaine Brown, was listed in serious condition early this morning at Children's Hospital where a team of doctors was still in the process of evaluating the extent of his injuries.

Older family members first learned of the accident when Kareem Smith, the child's 4-year-old brother, ran into the kitchen about 5 p.m. and told his teen-aged sister that Lamaine had toppled over the seventh floor balcony railing.

According to an aunt, Veronica Smith, at least two -- and possibly three -- children had been playing on the cramped, nine-foot-long balcony with a Big Wheel -- a plastic tricycle that Lamaine had received for his birthday last February. Barbara Smith, the child's mother, was in the bathroom when her son fell.

Barbara Smith broke into tears while her 15-year-old daughter, Laura, body was sprawled on the sidewalk on Delaware Avenue.

"The mother never did go downstairs," Veronica Smith said. "she was too upset. She went downstairs to a neighbor's house [apartment] until the police came and go her. When they called me, she was hollering and screaming. She kept saying, 'Why didn't someone watch him?'"

Doctors working feverishly over the child last night and early this morning expressed amazement that he was alive.

"They don't know why" he survived, said a hospital spokeswoman. He's just lucky."

An incomplete diagnosis early this morning indicated that Lamaine had suffered a broken left arm and shoulder bone, possible internal injuries and a head injury that "right now doesn't appear to be serious," the spokeswoman said.

She said the child was semi-conscious, able to recognize his mother and answer questions by nodding yes or no.

As half the family prepared to stand an all-night vigil at the hospital, Kareem and his sister Avis, 14, sat at home with friends awaiting word of Lamaine's condition.

Kareem, dressed in shorts and a polo shirt, sat quietly on the sofa sucking a grape Popsicle. The kitchen door leading to the balcony was closed, creating a stuffy warmth in the two-bedroom apartment.

Leading a visitor to the balcony, Avis pointed to the approximately 3 1/2-foot-high metal railing that Lamaine apparently had climbed. He had pushed his feet through the checkerboard metal grillwork and leaned over the rail to look out at the street below.

The wider portion of the balcony railing, facing an inner court, stood approximately 5 1/2-feet high. The bottom portion was stone, with a two-foot metal grill, like the smaller grill, topping it.

In the corner of the balcony rested the multicolored Big Wheel and a ragged, green screen door.

According to the family, the screen door that should have closed off the balcony had been broken for the last two of the nine years the family has lived in the $70-a-month apartment, so they had to open the wooden door to let in a breeze.

Two years ago Kareem had climbed the balcony railing, family members said, to wave to pedestrians below. An aunt saw him and screamed for him to get down.

"I just hope now they give her another place somewhere on a lower floor," said Veronica Smith. "She's been asking for another place. She asked two years ago for them to fix the screen door and they haven't."