27-Hydroxycholesterol (27OH-Chol) is of potential diagnostic interest due to its role in maintaining whole-body cholesterol homeostasis. Dopant assisted-atmospheric pressure photoionization (DA-APPI) has improved the sensitivity of 27OH-Chol analysis, in comparison to the published LC-APCI-MS method, allowing quantification from a very low amount of sample (<or=50 microL plasma). The method was validated for quantification from 50 microL and 15 microL plasma, with the limit of quantification (LOQ) of 10 ng/mL and 40 ng/mL plasma, respectively. A further advantage is that no prior derivatization was needed, unlike the LC-ESI-MS or the standard GC-MS method. The method validation also resulted in good linearity and recovery of 91-106%. The within-day and between-day coefficient of variation were less than 15% while giving the accuracy of 90.9-113.4%. This report summarizes the effects of some critical parameters on the ionization of 27OH-Chol using APPI, as well as the validation of the method. The sensitivity achieved with DA-APPI broadens the usefulness of the LC-MS method in clinical applications.