To present our observation of the development of a rare nail deformity in the prolonged course of disease of a critically ill patient with a pulmonary abscess.Case report.Tertiary referral, 16-bed, level I surgical ICU in an academic hospital.A 48-year-old Caucasian male was treated with penicillin for a pneumococcal meningitis and pneumonia. He developed a large pulmonary abscess of the right upper lobe and needed prolonged mechanical ventilation. Extensive surgical treatment was successful eventually. A remarkable feature concerned the occurrence of onycholysis of all finger nails and toe nails resulting in complete shedding of the nails (onychomadesis). This phenomenon can be regarded as an extreme manifestation of Beau's lines precipitated by a severe systemic insult.We observed the development of onychomadesis in a critically ill patient with a large pulmonary abscess. This association has not been described before.