<p>A ‘hundred-columned’ structure is twice mentioned by Martial (2.14.9: <i>centum pendentia tecta columnis</i>, 3.19.1: <i>proxima centenis ... columnis</i>), who places it in the vicinity of the *Porticus Pompei (its groves of plane trees are mentioned at 2.14.10, perhaps also at 3.19.2). Just N of Pompey’s Portico was a long structure labeled [HECAT]OSTYLVM on the Severan Marble Plan (Rodríguez Almeida, <i>Forma</i> pl. 32, frag. 39). Its Latinized Greek name corresponds well with Martial’s <i>centum columnae</i>. The <i>Chronicon</i> of St. Jerome confirms that it stood near the *Theatrum Pompeium, since both burned in A.D. 247 (Hieron., <i>Chron. a Abr.</i> 2263: <i>theatrum Pompei incensum et &lt;h&gt;ecatonstylon</i>). Coarelli (<i>LTUR</i>) identifies the Hecatostylum with the 1st-c. B.C. <i>porticus Lentulorum</i> (accepted as a strong possibility by Orlandi), though the present map keeps the conventional name. From the Marble Plan, it seems that this structure followed the present Via del Sudario, extending from the <i>cavea</i> of the Theatrum Pompeium as far as Via S. Nicola de’ Cesarini, the E limit of the *“Area Sacra” in Largo Argentina. Martial’s description (2.14.9) of the Hecatostylum as a long roofed colonnade is consistent with the Marble Plan, which shows columns divided into two rows (more numerous on the S row) separated by a continuous line. It appears that the Hecatostylum was physically joined to the Porticus Pompeianae, given the <i>exedrae</i> facing both N and S along the dividing line between the two, as shown on plans by Lanciani (<i>FUR</i> pl. 21), Gleason, and Coarelli (1997). The E end of the Hecatostylum has been excavated by Marchetti-Longhi; plans of this excavated portion form the basis for our map.</p>