By employing high-temperature superconducting quantum interference device ͑SQUID͒ magnetometers, we have assembled a second-order gradiometer for magnetocardiography ͑MCG͒ in unshielded environment. With this high-temperature superconductor ͑HTS͒ SQUID system, we demonstrated its diagnostic relevance for MCG in terms of signal-to-noise ratio, spatial resolution, frequency bandwidth, rejection of environmental disturbances, and long-term stability. The electronically balanced gradiometer consists of three HTS radio-frequency SQUIDs with superconducting coplanar resonators, mounted in axial gradiometric arrangement with a baseline of 7.5 cm. The system achieves a common mode rejection for axial homogeneous fields of about 10 4 without any mechanical balancing, and a white noise about 130 fT/ͱHz at 77 K, with an 8ϫ8 mm 2 flux pickup area. MCG maps above volunteers' chests have been recorded in unshielded environment in a bandwidth of about 130 Hz. We showed the influence of several notch filters ͑suppressing the power line frequency͒ on the quality of the MCG signals. © 2000 American Institute of Physics. ͓S0003-6951͑00͒03307-6͔Magnetocardiographic ͑MCG͒ measurements using superconducting quantum interference device ͑SQUID͒ sensors are usually performed in magnetically shielded rooms, to reduce the influence of electromagnetic disturbances from the environment. 1,2 However, the high cost of such a room represents a major economic obstacle for the widespread application of magnetocardiography. Therefore, MCG gradiometer systems operating in unshielded environment would be highly desirable.High-temperature superconductor ͑HTS͒ SQUID gradiometers of adequate sensitivity ͑magnetic field resolution͒ and good disturbance rejection could offer a lower system and operating cost than their low-temperature ͑LTS͒ equivalents. Until present, first-and second-order HTS SQUID gradiometers, measuring axial or tangential fields, were demonstrated and MCG data recorded without magnetic shielding. [3][4][5] In these measurements, low-pass filters with a cutoff frequency of below 30 Hz were used to reduce the power line interference at 50 Hz, or a 60 Hz notch filter was used. 5 However, the frequency components of the MCG higher than 50 and 60 Hz contain significant information. 2,6 For clinical diagnostics, a MCG system has to have a bandwidth wider than 100 Hz, so that further development is warranted. Electrocardiographic ͑ECG͒ instruments have bandwidths of 100 Hz or more.In addition, a diagnostically meaningful map of magnetic field above the patient's chest has to be constructed of many sensing points ͑e.g., arranged in a 6ϫ6 grid͒. Hence, a sufficient long-term stability of measurement is required if sequential mapping is necessary. This is especially critical when using a single measuring point system. In this case, mapping can take, typically, about 40 min.In this letter, we present an improved second-order HTS SQUID gradiometer addressing the requirements listed above. To demonstrate the diagnostic relevance of this system,...