BACKGROUND: Primary heart tumors in fetuses are rare and mainly represent rhabdomyomas. The tumors have a variable expression and can be associated with arrhythmias, including both wide and narrow QRS tachycardia. Although multiple Doppler techniques exist to assess fetal heart rhythm, it can be difficult to record precise electrophysiological abnormalities in fetal life. OBJECTIVE: Investigations defining precise electrophysiological diagnosis were performed by using fetal magnetocardiography (fMCG). METHODS: In addition to routine fetal echocardiography, fMCG was used to investigate electrophysiological rhythm patterns in a series of 10 fetuses with cardiac rhabdomyomas. RESULTS: The mean gestational age of the fetuses was 28.6 +/- 4.7 weeks. The multiple rhabdomyomas were mainly located in the right and left ventricles as well as around the atrioventricular groove. Arrhythmias or conduction abnormalities were diagnosed in all 10 patients, although only 6 of them were referred due to that indication. Remarkably, 80% (8 of 10) had associated Wolff-Parkinson-White pre-excitation. In addition, we found prominent P waves in 4 fetuses. CONCLUSION: In fetuses with rhabdomyomas, a disease where rhythm pathology is common, precise electrophysiological diagnosis can now be made by fMCG. fMCG is complimentary to echocardiography for rhythm assessment and can detect conduction abnormalities that are not possible to diagnose prenatally with M-mode or pulsed Doppler ultrasound. Risk factor assessment using fMCG can support pregnancy management and postnatal treatment and follow-up.