The P wave of the magnetocardiogram (MCG) was investigated in normal subjects and patients with mitral stenosis to determine its characteristics in normal conditions and left atrial overloading (LAO) and to analyze atrial activation by a magnetic field. In normal subjects, the MCG P wave was negative in left parasternal sites and positive in right lower sternal sites. The current source deduced from the MCG pattern and isomagnetic map was directed inferiorly and to the left through the entire phase of atrial activation, suggesting that in most normal cases the P wave reflects right atrial activity. In patients with mitral stenosis, a negative-positive biphasic P wave was seen more frequently than in normal subjects in left parasternal sites (p less than 0.005). In the late phase of atrial activation, the current source deduced from the isomagnetic map was shifted superiorly and to the left, suggesting an increased leftward force due to LAO. The MCG was similar in sensitivity to the ECG, for diagnosis of LAO, but in a few cases LAO could be detected from the MCG but not the ECG. These findings suggest that the MCG is clinically useful for diagnosis of LAO.