A patient is reported with progressive pulmonary fibrosis, who suffered a spontaneous pneumothorax which resulted in an immediate relief of breathlessness. The possible pathways involved are discussed. On account of this discussion it is concluded that in this case the proprioceptive somatic pathway was the most important one. The authors are inclined to extrapolate this conclusion to all those cases of breathlessness, in which the muscles of breathing meet with a pressure volume inappropriateness.