With all 51 precincts reporting in the 4th District, Republican challenger Thomas Muratore had 51 percent and Brian Beedenbender (D-Centereach) had 49 percent.
Speaking at Democratic headquarters in Hauppauge, Democratic Presiding Officer William Lindsay said: "It looks like we've lost one seat. We're not happy about it. It's not the end of the world. Tomorrow we'll wake up and pick up the pieces."
Beedenbender did not concede late last night.
"There's at least 600 absentee ballots out there and we'll see what happens," he said.
Suffolk GOP committee chairman John Jay LaValle said Muratore's apparent win was the biggest surprise of the night.
"The party is unified and we're very excited about the future," he said.
The legislature currently tilts Democrat 11-6, with another member who caucuses with neither major party.
With all precincts reporting in the 7th District, Legis. Jack Eddington (I-Medford), who caucuses with the Democrats, appeared to have held onto his seat with 52 percent of the vote, while his Republican challenger, L. Dean Murray, had 48 percent.
And with all districts reporting in the 16th District, Democratic Legis.
Steve Stern, 41, of Dix Hills, squeaked by with 52 percent, with Republican William Matthew Groh at 48 percent.
A key battleground was the open seat in the 10th District, where Republican Thomas Cilmi, a former legislative aide and Bay Shore businessman, beat Democrat Patrick Nolan, son of Islip Supervisor Phil Nolan.
Timothy Molnar, of Islip Terrace, the Conservative Party candidate, was in third place.
"It's a tough year to be a Democrat," said Patrick Nolan, who conceded.
In another hard-fought race, in the 11th District, Legis. Thomas Barraga, the veteran Albany and Suffolk lawmaker who lost two minor party primaries, beat his challenger, Democrat James Alcus.
Here's a rundown of the other races:
Incumbent Jay Schneiderman, 47, an Independence Party member from Montauk, was running unopposed on the Democratic, Republican, Independence, Conservative and Working Families lines, ran unopposed.
Republican Daniel Losquadro, the legislature's minority leader, beat Shoreham businessman Bryan Lilly, a Democrat.
Democratic Presiding Officer William Lindsay beat Matthew Silecchia, 25, a Conservative from Sayville running on the GOP line.
Ricardo Montano, 59, of Central Islip, was unopposed.
Republican Legis. John M. Kennedy Jr. beat Errol D. Toulon Jr.
Democratic Legis. DuWayne Gregory, of Amityville, 39, beat Republican Edward White Jr., 51, of Copiague. In July 2008, White mounted an unsuccessful bid against Gregory for the same seat in a special election.
Democratic Legis. Jon Cooper, the legislature's majority leader, beat Republican Frank J. Stalzer, 52, of Huntington.
With Rick Brand and Thomas Maier