Cassel v Superior Court involved the mediation of a business dispute in which the plaintiff, Michael Cassel, agreed to accept a settlement of $1.25 million dollars for his claims. Cassel then sued his attorneys for malpractice, breach of fiduciary duty, fraud and breach of contract claiming his attorneys coerced him to settle for an amount lower than he told his attorneys he would accept and for less than the case was worth. Prior to trial, Cassel's attorneys moved to exclude all evidence of private attorney-client discussions both immediately preceding and during the mediation on the topics of mediation settlement strategies and the attorneys' efforts to persuade Cassel to reach a settlement. The trial court granted this motion, but the Court of Appeal vacated the trial court's order. The California Supreme Court subsequently reversed the judgment of the Court of Appeal and reinstated the trial court's exclusion order.

Evidence Code Section 1119 governs the admissibility of oral and written communications during mediation. In pertinent part, Section 1119 states "all communications, negotiations, or settlement discussions between participants in the course of a mediation…shall remain confidential." The California Supreme Court held that the term "participants" was not limited to the actual parties to the mediation (that is, the litigants themselves), but rather included all participants, including the attorneys representing the litigants. As such, communications between a mediation party and his or her own counsel are covered by Section 1119 and inadmissible in malpractice actions. The Court stressed it was interpreting Section 1119 and invited the Legislature to reconsider the decision if it disagreed. The Court also refrained from specifically delineating what communications are made "for the purpose of, in the course of, or pursuant to, a mediation."