Given the preliminary results of these in vitro studies, we propose a clinical pilot investigation of the relationship between vitamin D levels and cathelicidin production in TB patients. It is well known that the majority of TB patients are deficient in vitamin D, but it is unknown whether the deficiency has functional consequences for treatment response and relapse rates. While recent in vitro data suggests that vitamin D supplementation of recent TB contacts does improve their immune response to Mycobacterium tuberculosis as measured in whole blood assays, clinical trials of vitamin D supplementation in latent or active TB disease have not been undertaken. In preparation for larger trials that would be needed to address this clinical question, we propose that the clinical relationship between physiologic levels of vitamin D supplementation and effect on cathelicidin production within the host needs to be studied, given recent suggestions from in vitro studies that ability to generate adequate cathelicidin levels may have an impact on quality and effectiveness of host immune defense against M. tuberculosis.