A method for determining the amount of an insulin-like growth factor-I (IGF-I) protein or of an insulin-like growth factor-II (IGF-II) protein in a serum or plasma sample, the method comprising: subjecting IGF-I protein or IGF-II protein in the sample to ionization by electron ionization, chemical ionization, electrospray ionization, photon ionization, atmospheric pressure chemical ionization (APCI), photoionization, atmospheric pressure photoionization (APPI), fast atom bombardment, liquid secondary ionization, matrix assisted laser desorption ionization (MALDI), field ionization, field desorption, thermospray/plasmaspray ionization, surface enhanced laser desorption ionization (SELDI), inductively coupled plasma, or particle beam ionization to produce ions detectable by mass spectrometry; and (i) for determining the amount of IGF-I protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry with a mass analyzer capable of a resolving power of full width at half maximum (FWHM) of greater than or equal to 10,000 and an accuracy of less than or equal to 50 ppm; wherein the amount of the determined ion or ions is related to the amount of the IGF-I protein in the sample; or (ii) for determining the amount of IGF-II protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry, wherein the amount of the determined ion or ions is related to the amount of the IGF-II protein in the sample. subjecting IGF-I protein or IGF-II protein in the sample to ionization by electron ionization, chemical ionization, electrospray ionization, photon ionization, atmospheric pressure chemical ionization (APCI), photoionization, atmospheric pressure photoionization (APPI), fast atom bombardment, liquid secondary ionization, matrix assisted laser desorption ionization (MALDI), field ionization, field desorption, thermospray/plasmaspray ionization, surface enhanced laser desorption ionization (SELDI), inductively coupled plasma, or particle beam ionization to produce ions detectable by mass spectrometry; and (i) for determining the amount of IGF-I protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry with a mass analyzer capable of a resolving power of full width at half maximum (FWHM) of greater than or equal to 10,000 and an accuracy of less than or equal to 50 ppm; wherein the amount of the determined ion or ions is related to the amount of the IGF-I protein in the sample; or (ii) for determining the amount of IGF-II protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry, (i) for determining the amount of IGF-I protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry with a mass analyzer capable of a resolving power of full width at half maximum (FWHM) of greater than or equal to 10,000 and an accuracy of less than or equal to 50 ppm; wherein the amount of the determined ion or ions is related to the amount of the IGF-I protein in the sample; or (ii) for determining the amount of IGF-II protein, determining the amount of said ions by high resolution / high accuracy mass spectrometry,