Psychiatric consultants are increasingly asked to evaluate the competency of medically ill patients, but the absence of accepted guidelines and gaps in the law can make their task difficult. A model procedure for evaluating competency developed by the authors stresses the importance of conducting a complete clinical evaluation of the patient, prescribes interventions to clarify the role of the consultant for referring physicians, outlines medical and legal principles governing competency, and identifies the requirements of the consultant's note. Consulting psychiatrists cannot declare a patient legally incompetent; they can only give an opinion about the patient's apparent competency. They can withhold their opinion, however, until they have treated a psychiatric condition that may be affecting the patient's competency. Case vignettes illustrate the application of the guidelines.