The 294 cases of Pseudomonas aeruginosa isolated at S.N.U.H. during a period of 18 months from January 1975 to June 1976 were analyzed on the age and sex of patients, sources of specimens, species and frequency of mixed micro-organisms, and the results of antibiotic sensitivity test. The following results and conclusions were made. 1) Ratio of male to female patients was 7.1 ; 2.9, average age of patient is 39 year-old. There were 9 patients over 70 year-old and 4 patients less than one year old. 2) Source of specimens were ; 113 cases of pus (38%), 68 cased of urine(23%), 48 cases of sputum (16%), 35 cases of bronchial aspiration(12%), 9 cases of blood cultures (3%), and others 14 cases (5%). 3) Frequencies of mixed microorganisms were ; 40 cases of coliform bacilli, 28 cases of non-sugar fermenting microorganism (5 groups in TSI reaction), 27 cases of Klebsiella, 16 cases of Coagulase (+)Staphylococcus, 15 cases of paracolon bacilli, 11 cases of Proteus species, 9 cases of E coli, and 25 cases other microorganisms. 4) Frequencies of single or mixed microorganism isolation were ; Pseudomonas aeruginosa alone : 159 cases, one species mixed : 113 cases, two species mixed : 18 cases, and three species mixed : 4 cases. 5) Monthly or seasonal frequency in isolation rate was not demonstrated. 6) In the antibiotic sensitivity test, Pseudomonas aeruginosa was sensitive to only gentamicin.