The costumes, made in China, had small buttons that contain 11 times the legal level of lead.
The shipment was one of the targeted loads diverted from the Port of Seattle to a U.S. customs warehouse to be opened and inspected for safety.
"There's a variety of reasons that we target," said Doug McBride of U.S. customs. "Previous violators would be one. Country of origin would be another, destination of the product, who the product's going to in the United States."
Last year, customs agents stopped more than 11,000 shipments of lead-contaminated products.
"You go home at the end of the week knowing you made a difference, made an impact and kept some child from getting sick, or prevented someone's house from burning down," said Craig Mabie of the Consumer Products Safety Commission.