It has been suggested that the molar ratio of octadeca 9,11 dienoic acid to linoleic acid in biological material provides an index of activity along the non-peroxide pathway of a free radical attack on polyunsaturated fatty acids. In 17 adults with cystic fibrosis the 'molar ratio' in nasal epithelial cells--a recognised target of the disease--exceeded that in 20 controls (median 2.09%, range 1.70-3.01% versus 1.56, 0.92-2.23%, p = 0.0002). The difference was also apparent, although less stark, upon analysis of serum in a further 22 CF patients (2.48%, 1.60-5.24%) and 25 controls (1.96%, 0.81-3.90%, p = 0.0348). There was no correlation between the 'molar ratio' and blood white cell count or erythrocyte sedimentation rate, severity of lung or liver disease, indicating that the raised values are a primary feature, rather than reflecting disease severity. Aberrant free radical activity may underlie cellular dysfunction in cystic fibrosis.