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 Colorado, 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 lawyer in the firm—or from a lawyer's previous firm—has represented a party adverse to the firm's current client on a substantially related matter. Disqualification can also occur when a lawyer's representation of a new client is substantially related to and directly adverse to the interests of a current client, or if the representation is compromised by the lawyer's responsibilities to another client, a third party, or the lawyer's own interests. These rules are grounded in professional ethics, particularly the protection of client confidences and the attorney-client privilege. In Colorado, the rules for attorney disqualification are derived from the Colorado Rules of Professional Conduct, case law, and other relevant statutes. Courts have discretion in applying these rules and may use them as guidelines rather than strict standards for disqualification.