Militants bombed adjacent houses belonging to two Shiite Muslim brothers south of Baghdad early on Wednesday morning, killing 18 members of their families, security and medical officials said.
The bombs targeted the houses in the town of Latifiyah, just south of Baghdad, shortly after midnight, an army officer and a doctor at a nearby hospital said.
Among the dead were at least five women and six children.
Latifiyah lies within a confessionally-mixed region south of Baghdad known as the "Triangle of Death", so named for the brutal violence that plagued the area during the peak of Iraq's sectarian war in 2006 and 2007.
Last week, another attack on a Shiite Muslim family in the town killed at least five people.
Violence has surged across Iraq in recent months to its highest level since 2008, sparking fears the country is slipping back into the all-out sectarian war that left tens of thousands dead in 2006 and 2007.