A method and a device (11) for monitoring a lift car door (5) in a lift system (1) are described. In this context, one or more distances is/are acquired on the basis of light (25) which is emitted inthe direction of the lift car door (5) by a distance-measuring device (13) and reflected at the lift car door (5) as a function of a current closed state of the lift car door (5). Subsequently, information about the current closed state of the lift car door (5) is derived solely by comparing the acquired distance with a previously acquired reference distance, and a signal which represents the information is fed, for example, to a remotely arranged control centre (27). A monitoring device (11) which is configured for this purpose can be installed as a retrofit component which is capable of working autonomously in existing lift systems (1), does not need to receive any data from a controller of the lift car door (5) or the lift system (1) and only requires relatively simple data processing means which only need to carry out a comparison of detected distance values with reference values.