On a big night for season finales, Fox’s “American Idol” and ABC’s “Lost” combined for about 50 million viewers Wednesday, with Fox winning the night comfortably across the board, according to preliminary nationals from Nielsen.

From 8 to 10, the fourth-season conclusion of “American Idol” averaged a big 12.2 rating/30 share in adults 18-49 and 29.4 million viewers overall, up slightly from last year’s finale and the show’s best numbers since its auditions round in January. ABC’s “Lost” similarly put up its biggest ratings since January (7.6 rating/19 share in adults 18-49, 20.2 million viewers overall), the net’s largest aud in the time period since February 2003.

Combined, the shows averaged 49.6 million viewers and commanded roughly half of the 18-49 audience watching television from 8 to 10.

In the evening’s final hour, NBC’s special off-night season finale of “Law & Order: Criminal Intent” (4.8/12 in 18-49, 14.5 million viewers overall) won out over ABC’s “Alias” season finale (4.4/11 in 18-49, 10.8 million viewers overall), with the latter winning in men 18-49 and adults 18-34.

CBS didn’t do much with its original telepic “Amber Frey: Witness for the Prosecution” (2.0/5 in 18-49, 7.5 million viewers overall).

Preliminary 18-49 averages for the night: Fox, 12.2/30; ABC, 6.5/16; NBC, 2.9/7; CBS, 1.7/4; UPN, 0.8/2; WB, 0.5/1.

In total viewers: Fox, 29.4 million; ABC, 17.1 million; NBC, 9.4 million; CBS, 7.0 million; UPN, 1.9 million; WB, 1.4 million.