Report on an epidemic of five cases of ophthalmia neonatorum caused by pseudomonas aeruginosa. The patients represented about 8% of the infants born and treated in one department during a period of six weeks. In four cases the ON was protracted in two patients it was complicated by dacryostenosis. At first all the patients were treated with locally administered chloramphenicol, to which pseudomonas aeruginosa was resistant. The three cases the serous secretion ended after opening of the lacrimal ducts together with local treatment with polymyxin, neomycin and gramicidin. In one case the pseudomonas aerginosa, together with S. aureus found in the secretion in vitro, was found to be sensitive to a combination of trimethoprim and sulfamethoxazole, given perorally, which terminated the secretion. The epidemic was evidently caused by the use of contaminated water in the nursery room.