A 122-channel neuromagnetometer with a helmet-shaped detector array covering the entire head allows simultaneous recording of magnetic fields over the whole cortex. The instrument has 122 planar first-order gradiometers in dual units at 61 measurement sites. The SQUIDs are directly coupled to the readout electronics, with amplifier noise cancellation to eliminate the need for separate preamplifiers inside the magnetically shielded room. We analyze the performance of the device and compare it with traditional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for equivalent current dipole fits, and information-theoretical channel capacity. Our analysis includes the fact that instrument noise is smaller than the background activity of the brain; the signal-to-noise ratio and the resolution of the planar array are in that case equal to or better than that of an axial array. The number of channels and their spacing are very suitable for neuromagnetic measurements. An example of recordings of auditory evoked brain activity, performed simultaneously over the whole cortex, is presented.