Two cases of apneustic breathing that had several unusual features, as compared with experimental apneusis, are reported here. Both patients had intact vagal function. Respiratory rhythm showed mainly inspiratory apneusis, with a relatively smooth background rhythm. Frequency and duration of apneusis either decreased or were unaffected by breathing CO2, and increased, in one case, after hyperventilation. Both patients were alert and conscious. One had a long history of narcotic abuse, but had no other neurological or respiratory abnormality. Another had been deaf and mute since an episode of meningitis in childhood and had had apneusis for over 35 years, but suffered no other neurological abnormality. These unusual features suggest that the pathogenic mechanisms for apneusis are not the same in experimental models and patients.