Very-thick, concentrated brushes of poly(methyl methacrylate) (PMMA) were synthesized by surface-initiated atom transfer radical polymerization in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, an ionic liquid (IL) solvent. Notably, the high polymerization rate in this IL produced 700 nm-thick PMMA brushes, while the grafting density was observed to increase with increasing feed concentration of a deactivator catalyst (Cu(II)Cl-2/ligand), suggesting that grafting density is significantly determined by chain growth during a single activation cycle. A concentrated PMMA brush, with a number-average molecular weight of 1.28 x 10(6), a polydispersity index of 1.23, a brush-layer thickness of 510 nm, and a grafting density of 0.30 chains nm(-2), was successfully synthesized by taking advantage of the IL effect that provides a higher propagation rate constant and a lower termination rate constant. Further increases in the brush-layer thickness were achieved by moderately pressurizing the polymerization medium (i.e., by the combined use of the IL solvent and pressure), and an approximately 1 mu m-thick concentrated polymer brush was conveniently synthesized at 50 MPa.