Internet-based cognitive behavior therapy (CBT) for severe health anxiety can be effective, but not all patients achieve full remission. Under these circumstances, knowledge about predictors is essential for the clinician in order to make reliable treatment recommendations. The primary aim of this study was to investigate clinical, demographic, and therapy process-related predictors of Internet-based CBT for severe health anxiety. We performed three types of analyses on data from a sample comprising participants (N = 81) who had received Internet-based CBT in a randomized controlled trial. Outcomes were a) end state health anxiety, b) improvement in health anxiety (continuous change scores), and c) clinically significant improvement. Outcomes were assessed at six-month follow-up. The results showed that the most stable predictors of both end state health anxiety and improvement were baseline health anxiety and depressive symptoms. Treatment adherence, i.e. the number of completed treatment modules, also significantly predicted outcome. Notably, health anxiety at baseline was positively associated with symptom improvement while depressive symptoms was negatively related to improvement. Demographic factors were largely without significant impact on end state symptoms or improvement. We conclude that baseline symptom burden and adherence to treatment have strong predictive effects in Internet-based CBT for severe health anxiety. Copyright © 2013 Elsevier Ltd. All rights reserved.