Soon after getting an offer to sell their 12-year-old audio book business in 1996, Michael and Deborah Raffin Viner retained corporate transactional lawyer Charles Sweet to negotiate a deal for them.

The Southern California couple made it clear that there were certain contractual conditions that were "essential and non-negotiable," and received Sweet's assurances that he had gotten them all included. He hadn't, so the Viners, claiming they lost millions, sued for malpractice, eventually winning more than $13 million at trial.

An appeals court reduced that amount by more than $5 million, but upheld the crucial finding that the Viners - in order to recover the value of what they lost - didn't have to prove that Sweet could have obtained a better outcome for them. In doing so, opponents say, the court hung transactional lawyers out to dry.

Last week, the California Supreme Court took up Viner v. Sweet during oral arguments in Los Angeles. Twenty-three California bar associations, law firms and individual lawyers have filed amicus curiae briefs with the court in support of Sweet, warning that transactional lawyers now face a vast new exposure to malpractice unless the lower court ruling is overturned.

Disgruntled clients, they say, can now sue lawyers for botched transactions without having to go through what's known as the "trial within a trial" required in malpractice cases arising out of litigation.

"The longstanding requirement of proof that the client would have achieved a better outcome absent negligence, the court of appeal held, applies only to litigation malpractice cases, not to cases alleging negligence in connection with a transaction," Mark Helm, who represents Sweet and his former law firm, Washington, D.C.'s Williams & Connolly, wrote in court papers. "This holding violates settled principles in the law not only of professional malpractice, but of negligence more generally, and it rests on fundamental errors of law and logic."

The Viners, long involved in the film and television industry, accused Sweet, now a partner at Honolulu's Carlsmith Ball, of neglecting to include seven specific conditions in the employment termination agreement that were essential to concluding a deal over their Dove Audio Inc. Among these were provisions providing the couple monthly payments from Dove for three years, credit for work done at the company and the right to engage in film and TV work - with the likes of Carl Reiner, Stephen King and Andy Rooney - that didn't interfere with Dove's audio book business.

Sweet had never drafted an ETA and ended up not including the conditions demanded by the Viners. Even so, Sweet and his firm argued that the Viners had the duty of proving at a trial within a trial that "but for" their lawyer's negligence they would have gotten a better deal.