An illumination system (20) for greenhouses comprises: a plurality of lamp units (101 ) comprising at least one LED load (12) and a driver (17) a central monitoring device (200) adapted for remotely monitoring at least one operational parameter of the lamp units and for generating an alerting signal (Sa) for service personnel when it detects that the monitored parameter is outside a predetermined range, A lamp unit may be provided with at least one sensor (22) for sensing the at least one operational parameter, and with communication means for communicating information to the central monitoring device. If a group of lamp units is supplied by a common mains supply line (201 ), the system may include a current sensor (203) arranged in or at the common supply line, for communicating to the central monitoring device a current sense signal (Sc) indicating the actual current flowing in the common supply line.