Although spin-echo techniques are often used to obtain 1H-NMR spectra of serum or plasma samples, they do not provide reliable quantitative analyses of metabolites. We present a standardized procedure, optimized for sensitivity, for using single-pulse 1H-NMR spectroscopy to analyze deproteinized plasma. The detection limit for various metabolites ranges between 2 and 40 mumol/L. The method allows quantitative analysis of many compounds of interest in studies of inborn errors of metabolism, including betaine and dimethylglycine, which cannot be measured easily with other techniques. For lactate, tyrosine, threonine, and alanine, we obtained results that correlated well with those obtained by established techniques. We also present a library containing resonance positions of 38 compounds occurring in plasma samples in health and disease, including 14 as-yet-unidentified resonances. As an example of the diagnostic power of the technique we show a spectrum of a plasma sample from a patient with 5-oxoprolinuria (pyroglutamic aciduria; McKusick 266130), an enzymatic defect in glutathione biosynthesis.