The use of extremely high pressures in liquid chromatography can improve the efficiency and reduce analysis time for columns packed with small particles. In this work, fused-silica capillaries with inner diameters of 30 microns are slurry packed with 1.5 microns nonporous octadecylsilane-modified silica particles. These columns are prepared in lengths up to 66 cm with packing pressures as high as 4100 bar (60,000 psi). Near the optimum flow rate, columns generate as many as 300,000 theoretical plates for lightly retained compounds (k' < 0.5) and over 200,000 plates for more retained compounds (k' approximately 2). These translate to plate heights (Hmin) as low as 2.1 microns. The pressures required to run at optimum flow rates are on the order of 1400 bar (20,000 psi). Analysis times at these pressures are on the order of 30 min (k' approximately 2) and can be reduced to less than 10 min at higher than optimum flow rates. Capacity factors are observed to increase linearly with applied pressure.