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Some confusion in the literature is clarified regarding the properties of microstrip‐line higher modes in the neighborhood of cutoff. It is shown that those modes become leaky in that range; with the aid of the steepest‐descent plane, one finds that the continuous spectrum can be characterized in a highly convergent manner by essentially a single leaky mode. The leakage occurs in two forms: a surface wave and a space wave. For structures without a top cover, it is found that almost all of the leakage is in the form of a space wave, so that an efficient leaky‐wave antenna of particularly simple configuration may be designed that consists of just a length of uniform microstrip line fed in its first higher mode. An accurate leaky‐wave analysis is developed that explains quantitatively the performance features and the limitations of this class of antennas.