The lawsuit on behalf of Smith, Ladell Betts, Anthony Davis, Christian Ballard and Gregory Westbrooks was filed Thursday in the U.S. District Court for the Eastern District of Missouri, with the plaintiffs claiming the NFL Players Association "withheld information from the players about the risks of head injuries."
The former players are seeking medical monitoring and financial compensation for long-term chronic injuries, financial losses, expenses and intangible losses. It refers to the "pathological and debilitating effects of traumatic brain injuries (TBI) caused by concussive and sub-concussive impacts."
The players named former union presidents Trace Armstrong, Troy Vincent and Kevin Mawae in the suit.
"This lawsuit has no merit and we will defend our union and our past presidents," the NFLPA said in a statement. "It erroneously alleges that the NFLPA knowingly and fraudulently concealed from players the risks of head injuries players faced by playing in NFL games and practices over the last several decades.
"The NFLPA has made the health and safety of its members a priority and the advancements in professional football on concussion education, prevention and treatment are a result of our efforts."
Smith spent 13 seasons in the NFL, nine with Kansas City, and was one of the game's top defensive players. He retired in 2000.
Betts was a running back for nine seasons, the first eight with Washington. He retired in 2010.
Davis played eight seasons with four teams and won a Super Bowl with Baltimore after the 2000 season, his last year in the league.
Ballard, a defensive end in 2011 and 2012, left the Vikings last September. Coincidentally, he is being represented by the union in a grievance concerning about $240,000 in 2013 salary that he collected but the team is trying to recoup.
Westbrooks, now 61, played parts of seven seasons from 1975-81 as a linebacker and special teamer with four clubs.