Context: Doctors have been known to treat or give consultation to patients informally, with none of the usual record keeping or follow up. They may wish to know whether this practice is ethical. Objective: To determine whether this practice meets criteria of medical ethics. Design: Informal medicine is analysed according to standard ethical principles: autonomy, beneficence and non-maleficence, distributive and procedural justice, and caring. Setting: Hospital, medical school, and other settings where patients may turn to physicians for informal help. Conclusion: No generalisation can be made to the effect that informal medicine is or is not ethical. Each request for informal consultation must be considered on its own merits. Guidelines: Informal medicine may be ethical if no payment is involved, and when the patient is fully aware of the benefits and risks of a lack of record keeping. When an informal consultation does not entail any danger to the patient or others, the physician may agree to the request. If, however, any danger to the patient or others is foreseen, then the physician must insist on professional autonomy, and consider refusing the request and persuading the patient to accept formal consultation. If a reportable infectious disease, or other serious danger to the community, is involved, the physician should refuse informal consultation or treatment, or at least make a proper report even if the consultation was informal. If agreeing to the request will result in an unfair drain on the physician's time or energy, he or she should refuse politely.