Perennials, (20-)40-70(-150) cm (slender, fibrous-rooted crowns, with slender rhizomes). Stems ascending to erect, puberulent to glabrate. Leaves opposite; petioles 2-20 mm; blades (3-5-nerved) lanceolate to lanceolate-ovate, mostly 3-6 × (1.5-)2-3 cm, bases obtuse to truncate, margins serrate to crenate, apices acute to acuminate, sparsely puberulent abaxially, mostly along nerves. Heads clustered. Peduncles 5-12(-20) mm, puberulent. Involucres 5-7 mm. Phyllaries: apices acute, abaxial faces glabrous or glabrescent, eglandular. Corollas white, lobes short-villous. Cypselae sparsely and finely strigose-hirsute. 2n = 85 [ca. 100, fide A. M. Powell on label]. Flowering (Jul-)Aug-Oct. Rocky slopes and ledges, in oak-juniper, pine-oak, pine, aspen, and spruce-fir woodland; 1700-2400 m; Ariz., N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Durango, Sonora). Ageratina rothrockii is similar to A. altissima, probably its western vicariant, and the two perhaps would be justifiably treated as conspecific.