A woman accusing Bill Cosby of sexual assault is entitled to any documents or emails the comedian’s criminal lawyer in Pennsylvania shared with a former prosecutor who is now a key defence witness.
The ruling on Friday comes in the defamation lawsuit that Andrea Constand filed against former prosecutor Bruce Castor.
Mr Castor had declined to charge Cosby over her 2005 police complaint. Mr Castor said last year that she enhanced her story when she sued Cosby.
Mr Castor has since said he made a binding deal that Cosby would not be arrested.
Cosby remains free on one million US dollars bail while an appeals court weighs that issue.
He is meanwhile locked in legal battles with Ms Constand and other accusers.
Cosby sued her this year, accusing her of violating their confidential settlement by talking to police.