We investigate an ordered-access protocol for cognitive radios. The primary user (PU) operates in a time-slotted fashion and starts transmitting at the beginning of the time slot if its queue is nonempty. The secondary users, depending on their queues and spectrum sensing results, may start transmitting at times <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex></formula> , <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{2}\tau$</tex> </formula> , <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{3}\tau \ldots$</tex></formula> relative to the beginning of the time slot, where <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex></formula> is the sensing duration. Secondary user <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$j$</tex></formula> is assigned rank or order <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$i$</tex></formula> and possibly starts transmitting at time <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$i\tau$</tex> </formula> relative to the beginning of the time slot with a certain probability designed to guarantee the stability of system queues. We consider two models. In the first model, which is denoted by <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> ${\cal S}$</tex></formula> , only one secondary user is assigned a particular rank, whereas in the second model, which is denoted by <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mathhat{\cal S}$</tex></formula> , there is less coordination, and some secondary users may have the same access order. We provide some analytical results for the case of two users and two ranks, under the assumption of perfect spectrum sensing. Our results show that system <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\cal S}$</tex></formula> is better than <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \mathhat{\cal S}$</tex></formula> in terms of the maximum stable throughput region. After considering perfect spectrum sensing, we provide outer and inner bounds on the maximum stable throughput region for the case of sensing errors. We then investigate the multiple-cognitive-user scenario. We prove the advantage of system <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mathhat{\cal S}$</tex></formula> over a random access scheme, where all the secondary users access the channel probabilistically after a sensing period of duration <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex></formula> , and the advantage of system <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\cal S}$</tex></formula> over a time-division multiple-access (TDMA) system, where each secondary user is individually assigned to a whole time slot for a certain fraction of the overall operational time and carries out spectrum sensing over duration <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex></formula> .