Carbon nanotubes are shaped like fibres(1) and can stimulate inflammation at the surface of the peritoneum when injected into the abdominal cavity of mice(2), raising concerns that inhaled nanotubes(3) may cause pleural fibrosis and/or mesothelioma(4). Here, we show that multiwalled carbon nanotubes reach the subpleura in mice after a single inhalation exposure of 30 mg m(-3) for 6 h. Nanotubes were embedded in the subpleural wall and within subpleural macrophages. Mononuclear cell aggregates on the pleural surface increased in number and size after 1 day and nanotube-containing macrophages were observed within these foci. Subpleural fibrosis unique to this form of nanotubes increased after 2 and 6 weeks following inhalation. None of these effects was seen in mice that inhaled carbon black nanoparticles or a lower dose of nanotubes (1 mg m(-3)). This work suggests that minimizing inhalation of nanotubes during handling is prudent until further long-term assessments are conducted.