Langerhans cell histiocytosis (LCH) is considered a non-hereditary disorder. Evaluation of the few familial cases might provide insight into its aetiology and pathogenesis. We conducted a survey to identify familial LCH cases. Data on family history, zygosity assessment in twins, clinical and laboratory features, treatment outcome, and present status were collected. According to variable confidence for twins monozygosity assessment, we termed these pairs 'presumed monozygotic' (pMZ). Nine families had more than one affected relative: five with LCH-concordant twin pairs, four with LCH in siblings or cousins. Three twin pairs not concordant for LCH were also studied. Overall, four of five pMZ twin pairs and one of three dizygotic (DZ) pairs were concordant for LCH. The pMZ twins had simultaneous and early disease onset (mean age 5.4 months); onset was at 21 months in the DZ pair. Clinical features were similar in the pMZ pairs. One pair of DZ twins had disseminated LCH. The three healthy twins (one pMZ, two DZ) remain asymptomatic 0.3, 5.9 and 4.7 years, respectively, after disease onset in their co-twins. Of the two families with affected non-twin siblings, one had known parental consanguinity and the other possible consanguinity. Potential consanguinity was also present in one of the two families with affected first cousins. Our data support high LCH concordance rates in pMZ twins and add the finding of LCH concordance in one of three dizygotic pairs studied. Taken together with our identification of LCH in siblings and first cousins from known or possibly consanguineous families, and with prior reports of three affected parent-child pairs, the data support a role for genetic factor(s) in LCH. The work-up of newly diagnosed patients should include a careful, extensive family history and chromosome studies. When possible, constitutional and/or lesional DNA should be obtained for future study.