Attorneys and their law firms may be disqualified from representing a client in a certain matter for a number of reasons. One of the most common reasons for an attorney’s disqualification is that the attorney previously represented another party (a former client) involved in a current dispute (and to whom the attorney’s current client is now adverse) on a matter that is substantially related to the current dispute.

For example, an attorney who represented one of the spouses in a divorce case would be disqualified from representing the other spouse in a subsequent motion to modify the divorce decree. And all the lawyers in a law firm may be disqualified from a case if one of the firm’s lawyers (or their former law firms) previously represented a party who is adverse to the new firm’s client on a substantially related matter. The same or similar rules may apply to paralegals.

A lawyer may also be disqualified from a new representation by the representation of a current client—if, for example, the new matter is substantially related and directly adverse to the interests of a current client of the lawyer or law firm; or (2) reasonably appears to be or become adversely limited by the lawyer’s or law firm’s responsibilities to another client; or to a third person; or by the lawyer’s or law firm’s own interests.

Disqualification of attorneys and motions to disqualify attorneys are usually grounded in professional ethics and the sanctity of client confidences—communications and information protected by the attorney-client privilege. A state’s rules of professional conduct or rules of disciplinary conduct will often be relevant to a motion to disqualify an attorney, but courts often treat these standards as guidelines and not as controlling standards for disqualification of attorneys.

Rules regarding the disqualification of attorneys vary from state to state and are often located in a state’s rules or code of civil procedure; court opinions (common law or case law); and statutes.

In Illinois, attorneys and their law firms may be disqualified from representing a client for several reasons, including conflicts of interest arising from previous representations. For instance, an attorney who has represented one spouse in a divorce cannot represent the other spouse in a related proceeding, such as a motion to modify the divorce decree. This principle extends to law firms, where disqualification can occur if any attorney in the firm has a conflict due to prior representation of an adverse party in a substantially related matter. Additionally, disqualification may be necessary if an attorney's representation of a new client is directly adverse to a current client, or if the attorney's duties to another client, a third party, or their own interests are compromised. These rules are rooted in the need to maintain professional ethics and protect client confidences, as governed by the attorney-client privilege. In Illinois, the relevant guidelines for attorney disqualification are found in the Illinois Rules of Professional Conduct, as well as in case law and statutes. Courts have discretion in applying these guidelines to determine whether disqualification is warranted in a given case.