The present invention concerns a method and a system for measuring non-invasively a blood pressure of a patient. The method comprises the steps of: determining (1603) a mechanical heart beat starting time point from an impedance cardiogram signal, detecting (1605) a heart beat pulse arrival time at a peripheral site of the patient, calculating (1607) a pulse wave transit time from the heart to the peripheral site by utilizing said mechanical starting point of the heart beat and said heart beat pulse arrival time,calculating (1609) an estimate of the blood pressure of the patient from said pulse wave transit time.