RESUMEN
BACKGROUND: The etiology and pathogenesis of microscopic colitis is unknown. Whether genetic predisposition is of importance, as in many other gastrointestinal diseases, is unknown. Familial occurrence of collagenous colitis has earlier been reported only in two families. METHODS: Familial occurrence of microscopic colitis was searched for in a Swedish national microscopic colitis register. RESULTS: Familial occurrence of microscopic colitis was identified in five families. In all families a sister-sister relationship was found. Two sisters with collagenous colitis had been living apart in different Nordic countries for many years before developing the disease. In one pair, the smoking sister had collagenous colitis and the never smoking sister had lymphocytic colitis. CONCLUSIONS: Considering the relative rarity of microscopic colitis, these findings indicate that a genetic predisposition may be of importance.