BACKGROUND: Fractional exhaled nitric oxide (FENO) is a sensitive marker of eosinophilic airway inflammation in asthma. Available methods have restricted measurements to the clinic, giving only a snapshot of the disease, which by nature is highly variable. OBJECTIVES: We sought to investigate the feasibility, repeatability, accuracy, sensitivity, and biologic plausibility of new handheld equipment for FENO measurements. We studied day-to-day home measurements of FENO during the birch pollen season in children with allergy to birch pollen and a history of mild asthma and rhinoconjunctivitis during this season, as well as in nonatopic children. METHODS: Eleven children with mild asthma and allergy to birch pollen, performed daily home measurements of FENO for 6 weeks before and during the birch pollen season by using a handheld FENO monitor (NIOX MINO). Additionally, FENO (chemiluminescence equipment [NIOX]) and spirometry were measured at the inclusion and completion visit in the clinic. Peak expiratory flow rate (PEFR) and symptoms were recorded daily. RESULTS: Daily FENO (NIOX MINO) increased significantly (P < .001) with increasing pollen count. FENO (NIOX MINO) and FENO (NIOX) exhibited a correlation coefficient of 0.98, but FENO (NIOX MINO) was significantly higher than FE(NO) (NIOX) (P < .01). PEFR and FEV1 remained unchanged, and few symptoms were recorded. CONCLUSION: Exhaled nitric oxide levels increased significantly during the pollen season, even though the patients reported only few asthmatic symptoms and no change in PEFR or spirometry. Daily measurements of FENO (NIOX MINO) might allow early detection of disease deterioration, and future studies could address such a measure for dynamic treatment strategies. CLINICAL IMPLICATIONS: This simple handheld device expands the potential use of FENO to a wider group of asthma clinics and even home measurements.