The pathogenesis of sarcoidosis is still largely unknown. The generally accepted theory is that genetically predisposed individuals develop the sarcoid disease reaction as a response to one or more unknown antigen(s). A single study by Nilsson et al has related the development of sarcoidosis to an infection with Rickettsia helvetica. The aim of this thesis was to investigate whether a rickettsial infection is involved in the pathogenesis of sarcoidosis. We used different microbiological methods as serology, polymerase chain reaction, and fluorescent in situ hybridization (FISH) on samples from patients with sarcoidosis and control patients. The thesis compiles the results from four separate studies: The second paper describes a serological survey in historical patient sera. None of the results from the studies supported the hypothesis of Rickettsia being involved in the pathogenesis of sarcoidosis. In conclusion, we could not find evidence to support the primary hypothesis of the study, that a rickettsial infection should be involved in the pathogenesis of sarcoidosis.