A precolumn derivatization method was optimized for rapid and specific analysis of total urinary hydroxyproline by HPLC. After an overnight hydrolysis, urine samples dried and reconstituted with the internal standard cysteic acid (in sodium hydrogen carbonate, pH 9.3) were derivatized with N,N-diethyl-2,4-dinitro-5-fluoroaniline (FDNDEA) at 100 degrees C for 20 min. The DNDEA-hydroxyproline adduct was separated on an Ultrasphere ODS column with a mobile phase of acetate buffer (containing triethylamine, 6 mL/L, pH 4.3) and acetonitrile (80/20, by vol), and was detected at 360 nm. A single run took 18 min with a hydroxyproline retention time of 7.3 min. The assay showed a linear response to hydroxyproline concentrations from 5 to 100 mg/L with a detection limit of 0.8 ng injected, corresponding to 2 mg/L in urine. Mean (SD) analytical recovery was 94.2 (13)% and 104 (9)% at 10 and 50 mg/L, respectively. Within-run and between-run CVs (n = 10) were 3.74% and 4.33%, respectively, for 25 mg/L. Results for samples (n = 50) analyzed by HPLC (y) vs ion-exchange chromatography with postcolumn ninhydrin reaction (x) correlated well: y = 0.98x + 1.02 (r = 0.985, Sxy = 3.13). In another comparison, involving 173 samples, a colorimetric procedure (Hypronosticon, x) gave slightly higher values than the HPLC method (y): y = 0.83x + 2.21 (r = 0.937, Sxy = 4.6).