Radio astronomy interferometers have potential use for systematic monitoring of medium-scale irregularities in the ionosphere. In this article, precepts for their use are derived, and preliminary analysis of data samples from interferometers at Green Bank, West Virginia, and Magdalena, New Mexico, is presented. A typical differential electron content of about 1015/m2 over a 35-km baseline is observed. A day-night correlation with TEC values is observed; we found the nighttime ionosphere to be more disturbed during high geomagnetic activity. Ionospheric irregularities are observed to persist over periods of an hour or longer.