King County was dropping 175 cases, prosecutor Dan Satterberg said. Pierce County prosecutor Mark Lindquist said his office will do the same, but he didn't immediately know the number of cases affected.
Under Initiative 502, possession of an ounce or less of marijuana will be legal for people 21 years or older after Dec. 6. The initiative passed Tuesday with 55 percent of the vote.
"Although the effective date of I-502 is not until Dec. 6, there is no point in continuing to seek criminal penalties for conduct that will be legal next month," Satterberg said in a statement.
The cases in King County involve people over 21 who possessed one ounce or less of marijuana. Dropping those pot cases won't ease caseload, said prosecutor's office spokesman Dan Donohoe, noting that 6,700 misdemeanor cases were tried in 2011.
Lindquist cautioned, however, that cases of possession of marijuana linked to other charges, such as drunken driving for example, would proceed. The cases affected are solely marijuana possession, he said.
In Snohomish County, chief criminal deputy prosecutor Joan Cavagnaro said her office is in a "holding pattern." A call to the Spokane County prosecutor's office was not immediately returned.
Along with Washington, Colorado legalized recreational use of marijuana Tuesday. A legalization measure was defeated in Oregon.
Besides de-criminalizing possession, Washington's initiative also sets up a state-run regulatory system that will license growing and stores to sell marijuana.
State officials, however, are awaiting a response on the initiative from the federal government, which still considers the drug illegal and could intervene in court.
Some legal experts believe that the de-criminalization provision in the law could survive a legal challenge.
"The parts that are more likely to be enjoined are probably the parts where you got states actually authorizing the growing, the sales -- with pretty direct conflict with federal law," said Mary Fan, a University of Washington law professor and former federal prosecutor.
Earlier this week, the Seattle Police Department said it was not arresting people for marijuana possession.
Seattle defense attorney Steve Karimi commended Satterberg for setting an example and following the will of the people on this issue.
Karimi, who says 15 to 20 percent of his cases involve marijuana, predicted prosecutors in smaller counties may not be as cooperative and forward-thinking.
Case law is not crystal clear on whether marijuana cases can still be effectively prosecuted after the law changes, he said, adding that most alcohol possession cases evaporated when Prohibition ended.
Misdemeanor possession cases may not seem like a big deal to the general public, but Karimi noted the results can be disastrous for people who have certain jobs, such as teachers. Immigrants also have been deported over possession convictions, he noted.
Associated Press writer Donna Gordon Blankinship contributed to this report.
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