Objective Anthroposophic painting therapy (APT) is a specific form of art therapy that aims to activate self-healing capacities through painting aquarelles. Methods The Anthroposophic Art Therapy Assessment-Paint’ (AART-ASSESS-P) was developed to measure pictorial expression and validated in the framework of a comprehensive cohort design study. The validation study examined 68 breast cancer patients with fatigue. Art therapists made pre- and post-assessments of spontaneously drawn water-color paintings with a preliminary version of the AART-ASSESS-P (58 items). Inter-rater reliability (IRR) for the items was examined with Cohen's weighted Kappa (κw). Additionally, a reliability- and factor analysis (FA) were conducted. Convergence criteria were patients’ self-report measures: the Satisfaction with Painting Therapy, Inner Correspondence with Painting Therapy and the Self-Regulation Questionnaire. Results IRR for the items was heterogeneous (κw= 0.09-0.89, Mean κw= 0.40, SD = 0.17). Thirty-six items were excluded due to insufficient IRR and item-total correlation (κw= Conclusion The AART-ASSESS-P is the first reliable instrument to measure pictorial expression during APT.