Mendelian susceptibility to mycobacterial disease (MSMD) is characterized by infections with weakly virulent mycobacteria (BCG and environmental mycobacteria), M. tuberculosis, Salmonella, candida and some other intracellular microorganisms. Nine different genetic defects have been defined to cause MSMD and IL-12R beta 1 deficiency is the most common form. We present here the clinical and genetic features of 18 patients with IL12R beta 1 deficiency diagnosed by surface expression of IL-12R beta 1 and Sanger's sequencing. Seventeen patients showed classical presentation (infections with BCG, salmonella and candida) while one patient experienced recurrent leishmaniasis. In all patients the percentage of activated lymphocytes with surface expression of IL12R beta 1 was <1% indicating that it is an effective method for the screening of these patients. Three recurrent mutations were responsible for 85% of our families. Prognosis was good in patients, in whom specific antimicrobial therapy was given before dissemination occurs, as well as prophylactic antimicrobial treatment when needed and IFN-gamma therapy for severe infectious episodes.