A detectably labeled HSP90 inhibitor for use in a method for determining whether a tumor will likely respond to therapy with an HSP90 inhibitor which comprises either the following steps: (a) contacting the tumor with a detectably labeled HSP90 inhibitor which binds preferentially to a tumor-specific form of HSP90 present in a tumor or tumor cells; (b) measuring the amount of labeled HSP90 inhibitor bound to the tumor; and (c) comparing the amount of labeled HSP90 inhibitor bound to the tumor measured in step (b) to a reference amount of the labeled HSP90 inhibitor bound to normal cells; wherein a greater amount of labeled HSP90 inhibitor bound to the tumor measured in step (b) as compared with the reference amount indicates the tumor will likely respond to therapy with an HSP90 inhibitor; or which comprises the following steps: (a) administering a detectably labeled HSP90 inhibitor, wherein the detectably labeled HSP90 inhibitor is a radiolabeled form of the HSP90 inhibitor which binds preferentially to a tumor-specific form of HSP90 present in a tumor or tumor cells, to the patient at multiple time points within the period during which the patient is receiving therapy with an HSP90 inhibitor; and (b) measuring the concentration of the radiolabeled HSP90 inhibitor in the patient's tumor at such multiple time points after the administration in step (a); and (c) comparing the concentrations of the radiolabeled HSP90 inhibitor measured in step (b) with the minimum concentrations of the HSP90 inhibitor needed to effectively treat the tumor, wherein measured concentrations greater than the minimum needed to treat the tumor indicate that the patient is likely to respond to therapy with the HSP90 inhibitor; wherein in both of the above mentioned sequences of steps the detectably labeled inhibitor is: or a pharmaceutically acceptable salt thereof, wherein: X 3 is CH 2, CF 2, S, SO, SO 2, O, NH, or NR 2, wherein R 2 is alkyl; X 2 is 123 I, 124 I, 125 I, or 131 I; X 4 is hydrogen or halogen; X 5 is O or CH 2; R is 3-isopropylaminopropyl, 3-(isopropyl(methyl)amino)propyl, 3-(isopropyl(ethyl)amino)propyl, 3-((2-hydroxyethyl)(isopropyl)amino)propyl, 3-(methyl(prop-2-ynyl)amino)propyl, 3-(allyl(methyl)amino)propyl, 3-(ethyl(methyl)amino)propyl, 3-(cyclopropyl(propyl)amino)propyl, 3-(cyclohexyl(2-hydroxyethyl)amino)propyl, 3-(2-methylaziridin-1-yl)propyl, 3-(piperidin-1-yl)propyl, 3-(4-(2-hydroxyethyl)piperazin-1-yl)propyl, 3-morpholinopropyl, 3-(trimethylammonio)propyl, 2-(isopropylamino)ethyl, 2-(isobutylamino)ethyl, 2-(neopentylamino)ethyl, 2-(cyclopropylmethylamino)ethyl, 2-(ethyl(methyl)amino)ethyl, 2-(isobutyl(methyl)amino)ethyl, or 2-(methyl(prop-2-ynyl)amino)ethyl; and n is 1 or 2. (a) contacting the tumor with a detectably labeled HSP90 inhibitor which binds preferentially to a tumor-specific form of HSP90 present in a tumor or tumor cells; (b) measuring the amount of labeled HSP90 inhibitor bound to the tumor; and (c) comparing the amount of labeled HSP90 inhibitor bound to the tumor measured in step (b) to a reference amount of the labeled HSP90 inhibitor bound to normal cells; (a) administering a detectably labeled HSP90 inhibitor, wherein the detectably labeled HSP90 inhibitor is a radiolabeled form of the HSP90 inhibitor which binds preferentially to a tumor-specific form of HSP90 present in a tumor or tumor cells, to the patient at multiple time points within the period during which the patient is receiving therapy with an HSP90 inhibitor; and (b) measuring the concentration of the radiolabeled HSP90 inhibitor in the patient's tumor at such multiple time points after the administration in step (a); and (c) comparing the concentrations of the radiolabeled HSP90 inhibitor measured in step (b) with the minimum concentrations of the HSP90 inhibitor needed to effectively treat the tumor, wherein measured concentrations greater than the minimum needed to treat the tumor indicate that the patient is likely to respond to therapy with the HSP90 inhibitor; X 3 is CH 2, CF 2, S, SO, SO 2, O, NH, or NR 2, wherein R 2 is alkyl; X 2 is 123 I, 124 I, 125 I, or 131 I; X 4 is hydrogen or halogen; X 5 is O or CH 2; R is 3-isopropylaminopropyl, 3-(isopropyl(methyl)amino)propyl, 3-(isopropyl(ethyl)amino)propyl, 3-((2-hydroxyethyl)(isopropyl)amino)propyl, 3-(methyl(prop-2-ynyl)amino)propyl, 3-(allyl(methyl)amino)propyl, 3-(ethyl(methyl)amino)propyl, 3-(cyclopropyl(propyl)amino)propyl, 3-(cyclohexyl(2-hydroxyethyl)amino)propyl, 3-(2-methylaziridin-1-yl)propyl, 3-(piperidin-1-yl)propyl, 3-(4-(2-hydroxyethyl)piperazin-1-yl)propyl, 3-morpholinopropyl, 3-(trimethylammonio)propyl, 2-(isopropylamino)ethyl, 2-(isobutylamino)ethyl, 2-(neopentylamino)ethyl, 2-(cyclopropylmethylamino)ethyl, 2-(ethyl(methyl)amino)ethyl, 2-(isobutyl(methyl)amino)ethyl, or 2-(methyl(prop-2-ynyl)amino)ethyl; and n is 1 or 2.