The unencrypted data on the CDs includes member names, home addresses, dates of birth, medical procedure codes, diagnosis codes and member ID numbers, and an unspecified number of Social Security numbers.

The discs had been put in a box and sent via certified mail to CalOptima by one of its claims scanning vendors, according to a statement by the health plan. But CalOptima received just the external packaging material, minus the box of discs, the statement said.

A CalOptima spokesman today said it's not clear how the box went missing, but added that there is nothing to suggest that it was stolen.

"As of today, the missing disks have still not been found," the spokesman said. CalOptima plans to offer credit monitoring services for all those affected by the breach. The organization is now negotiating an agreement with one of the three major credit monitoring bureaus, he said. Once that's done, CalOptima will begin sending out notification letters to those affected, the spokesman said.

The health plan also wants to find out why the third-party claims-scanning vendor did not encrypt the data, he said.