Sarcoidosis is a chronic, multiorgan inflammatory disease of unknown etiology. Genetic alterations in the microsatellite level have been described in benign and malignant diseases of the lung. In the present study we investigated the incidence of Microsatellite lnstability (MSI) and Loss of Heterozygosity (LOH) in sputum cytological specimens from 30 patients with sarcoidosis, using 10 highly polymorphic microsatellite markers located at several chromosomal arms. Specimens from 30 healthy, matched subjects were used as a control group. Our results showed that 14 (47%) sarcoidosis patients showed genetic alterations, either MSI or LOH. Six (20%) patients exhibited MSI and 9 (30%) exhibited LOH in at least one microsatellite marker. One of the patients exhibited MSI in two microsatellite markers and 3 (10%) showed LOH in more than one marker. One patient showed complete deletion of the chromosomal arm l7q11.2-q21. None of the healthy subjects exhibited any genetic alteration in the studied markers. No correlation was found between the genetic alterations detected and age, disease duration, blood gases or spirometric parameters of the patients. Our findings suggest that MSI is a detectable phenomenon in sarcoidosis and seems not to be related with the severity of the disease. The detection of LOH indicates the presence of putative tumor-suppressor genes at loci examined, which may play an important role in the aetiopathogenesis of sarcoidosis. Further studies are needed to detect a possible prognostic value of our findings in lung cancer development. Pneumon 2000, 13 (1): 64-72