Performance of CMS ECAL with first LHC data

In the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC), the high resolution Electromagnetic Calorimeter (ECAL), consisting of 75 848 lead tungstate crystals and a silicon/lead preshower, will play a crucial role in the physics program. In preparation for the data taking a detailed procedure was followed to commission the ECAL readout and trigger, and to pre-calibrate, with test beam and cosmic ray data, each channel of the calorimeter to a precision of 2% or less in the central region. The first LHC collisions have been used to complete the detector commissioning and will have to provide the first in situ calibration. In this talk the CMS ECAL status and performance with the first collisions delivered from LHC will be reviewed.