There is the need to determine the best nitrogen (N) fertilizer rates for maximum yields and optimum tuber quality of existing potato cultivars. Urea ammonium nitrate was applied at 0, 90, 157, 224 and 291 kg N ha^-1 in split applications during the 2003 and 2004 growing season in a high altitude irrigated soil of the San Luis Valley Research Center, Colorado. Yield, tuber size distribution, and quality of AC93026-9RU (9RU) and CO93001-11RU (11RU), advanced russet potato lines, were collected. Specific gravity was decreased with N rate for 9RU and increased for 11RU. No internal defects were observed in either cultivar with N fertilizer. Increasing N rates showed quadratic and/or cubic trends for total yield, marketable size tuber yield and culls in both cultivars, but they increased the percentage of external defects. This study clearly shows that the optimum N rate may be different depending on the Farmer's goals of maximizing yields without internal defects vs. optimal tuber quality (no external defects). These responses in yield, tuber size distribution and external tuber quality need to be considered in commercial production when deciding the N rate to use.