Elevated neurotrophin concentrations have been shown in nasal and bronchoalveolar lavage fluids as well as in the sera of patients with allergic rhinitis and asthma. Concentration of nerve growth factor correlated with disease severity, bronchial hyperreactivity, and levels of mediators released from eosinophils. Due to the release of cationic proteins, oxygen species, and cytokines after degranulation, eosinophils contribute to tissue damage and can influence airway hyperresponsiveness in asthma. It has been hypothesized that neurotrophins may be involved in the development of eosinophilia and in activation of these cells. The aim of this review is to elucidate the direct and indirect mechanisms of neurotrophins contributing to eosinophilia in allergic diseases.