The trial court abused its discretion in disqualifying defendant’s attorney on the ground the attorney would be defendant’s principal witness regarding a decade-old settlement agreement between the parties–one that the attorney had negotiated for defendant. When the client gives fully informed consent, as was true here, the attorney-witness may be disqualified only if the trial court finds prejudice to the opponent or court system outweighs the prejudice to the client. Here, the trial court misweighed that balance. The prejudice to the client from disqualification was extreme, given the attorney’s familiarity with the long-running litigation, while there was little prejudice to the opponent which had not sought to disqualify the attorney despite early knowledge that he had played a crucial role in negotiating the settlement and thus was a likely witness, and no particular prejudice to the judicial system beyond the factors applicable generally to all attorney-witness situations.