Of a total of 40 patients with discolored nails studied, 34 had definite green discoloration of one or more nails. Bacteriologic studies of 26 patients with green nails revealed the presence of Pseudomonas aeruginosa in pure or mixed culture in 23 cases. Nonpigmented onychopathic or normal nails from 50 patients were cultured with only one patient yielding Ps. aeruginosa. Of the 34 patients with green nails, 32 listed occupations in which excessive exposure to water, soap, and detergents is common, 11 patients presented evidence of mechanical trauma to the nail, and three had concurrent onychomycosis. Histologically, Gram-negative bacilli consistent with Ps. aeruginosa were demonstrated within the nail plate. It is postulated that Ps. aeruginosa was able to invade the nail plate because of a local reduction of resistance produced by trauma or concurrent pathology.