在大部分的內線交易案例中，公司董事、經理人等將公司內部尚未公開的重大資訊告知其兄弟姐妹、父母親及朋友。這些人再基於未公開的重大資訊買、賣股票，這是典型的內線交易。然而，當一個人偶然聽聞未公開的重大資訊，並基於該資訊買、賣股票，這樣的行為是否合法？美國與台灣，有許多關於偶然聽聞的司法判決。這些判決中，有判決認定被告有罪，也有判決宣告被告無罪。在本文當中，比較了美國、歐盟以及台灣對於內線交易的規範。本文發現，由於對內線交易的規範不同，因此導致偶然聽聞內線消息，並基於該消息買、賣股票者是否構成內線交易有不同的結論。在本文的結論，本文主張應該要正視美國與台灣對於內線交易規範的不同因此，源於美國法的信賴理論與私取理論不宜用來解釋台灣規範內線交易的法律，美國法的消息傳遞者與消息受領人之概念亦是如此。本文更主張對於台灣禁止內線交易法律中的消息受領人應採取最廣義的解釋，因此，偶然聽聞內線消息並基於該資訊買、賣股票之行為是違法的。

In most insider trading cases, the officer or director of a company disclose non-public material information to his siblings, parents, or friends. The person who receives inside information buys or sells stocks based on the non-public material information. Such cases are typical insider trading cases. However, when someone overhears non-public material information, he/she buys or sells stocks based on such information. Is it illegal? There are many judicial decisions which are related to overhearing cases in US and Taiwan. However, some judicial decisions find the defendant who overhears non-public material information and buys or sells stocks based on such information guilty. Some judicial decisions find the defendants innocent. In this article, it compares the regulations of insider trading among U.S., E.U. and Taiwan. It finds out that the differences of regulations for insider trading results in the difference answer to whether it is legal for someone who overhears non-public material information and buys or sells stocks based on such information. In the conclusion, the author figures out that we have to face the differences between the anti-insider trading laws in U.S. and Taiwan. Based on that, the fiduciary theory and misappropriation theory which arising out of U.S. anti-insider trading cases cannot be applied to interpret statutes regulating insider trading in Taiwan, neither does the tipper/tippee theory. The author advocates the best approach to interpret the meaning of tippee under Taiwan anti-insider trading regime is the broadest one. Therefore, it is illegal for a person who overhears the non-public material information and buys or sells stocks based on that information under Taiwan anti-insider trading regime.