December 2, 2007 -- A sperm donor who sent gifts signed "Dad" to his biological son has been slapped with a child-support order, 18 years after helping his friend get pregnant.

The Nassau County man donated his sperm to a work colleague, and included his name on the child's birth certificate, saying it would give the boy an identity, courts documents revealed.

He then blurred the lines between donor and full-time father by sending money, presents and cards signed "Dad" and "Daddy," and having phone chats with the now college-bound teen.

But the man's goodwill backfired: A court ruling says he is now liable for financial support of the 18-year-old, who lives with his mother in Oregon.

"It really is no good deed goes unpunished," said the man's lawyer, Deborah Kelly of Potrush and Daab in Garden City.

"When people do things they think are being done with good intentions and there is an agreement and one of the party reneges on the agreement, it is certainly disconcerting."

She said the time lapse was "unusual."

"He was assured that he would have no responsibility on his part and of course 18 years has elapsed where there hasn't been responsibility," she said.

"He did not anticipate this would happen now, when the child is almost an adult, that the mother would come forward for child support."

She said her client had requested a DNA test, "because we have no concrete evidence he is the father."

Nassau County Family Court judge Ellen Greenberg ruled Nov. 16 against a paternity test, saying it would have a traumatic effect on the child.

The child signed an affidavit stating that he has "never known anyone other than [the man] to be his father," according to court documents.

If payments were to go ahead, the child support would be determined based on the mother's earning capacity; the reported income of her partner, who is also a doctor; and the father's income.

Because of privacy concerns, all parties remain unnamed.

The mother is identified in court papers as P.D., the alleged father as S.K., and their son as K.K.

The donor was a married doctor at a Nassau County hospital when he donated his sperm to a hospital resident and her female partner in the late 1980s.

At the time of the boy's birth in 1989, the man orally agreed he would not have any rights or benefits in the child's upbringing.

The father said he had contact with the child from his birth until 1993, when the lesbian couple and his son moved to Oregon, according to court documents.

From then the contact dropped to seven phone calls in the past 15 years and one meeting for a few hours three years ago.

Calls by The Post to the mother's attorney, Jeffrey Herbst, were not returned.

About 1 million American children are the product of sperm donors - the majority of them being anonymous fathers - with 30,000 more born each year.

Court rulings over parental rights from artificial insemination remain murky. Similar cases across the country have varied.

The Washington State Court of Appeals ruled in 2004 that a donor isn't bound to pay child support, unless he and the mother have a signed contract.

But earlier this year, a Pennsylvania judge held a sperm donor liable for support, noting he had spent thousands of dollars on toys and clothing for two children that he helped a lesbian couple conceive.