In a series of 155 patients with decompensated liver cirrhosis spontaneous bacterial peritonitis (SBP) was diagnosed in 15 cases (9.7%) and portal thrombosis (PT) in 8 (5.1%) by mean of standard criteria. The main clinical and laboratory characteristics were similar in the two groups of patients; fever was more frequently recorded in the SBP patients. Cytological examination of the ascites showed an increase in total cell count and in neutrophils count higher in SBP group than in PT. The diagnosis of PT was confirmed by ultrasonography. The data suggest that a cytological examination of the ascites should be performed in all patients with decompensated cirrhosis and may contribute to the differential diagnosis of SBP or PT.