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Chapter 9 212 Abstract Introduction End-of-day symptom diaries are recommended by drug regulatory authorities to assess treatment response in patients with irritable bowel syndrome (IBS). We developed a smartphone application to measure treatment response. Because the employment of an application to measure treatment response in IBS is relatively new, we aimed to explore patients’ adherence to the diary and characteristics associated with adherence. Methods A smartphone application was developed to serve as a symptom diary. IBS patients (based on Rome IV criteria) were instructed to fill out end-of-day diary questionnaires during the eight-week treatment. Additional online questionnaires assessed demographics, IBS symptom severity, and psychosocial comorbidities. Adherence rate to the diary was defined as the percentage of days completed out of total days. Adherence to the additional web-based questionnaires was also assessed. Results Overall, 189 patients were analyzed (mean age 34.0±13.3 years, 77.8% female). The mean adherence rate to the diary was 87.9±9.4%. However, adherence to the diary decreased over time ( P <0.001). No significant association was found between adherence and age, gender, or educational level, while higher anxiety scores were associated with lower adherence ( P =0.03). Adherence to the online questionnaires was also high (>99%). Missing data due to technical issues was limited. Conclusions The use of a smartphone application as a symptom diary to assess treatment response resulted in high patient adherence. The data-collection framework described led to standardized data-collection with excellent completeness and can be used for future RCTs. Due to the slight decrease in adherence to the diary use throughout the study, this method might be less suitable for longer trials.