In order to verify whether quantitative morphological indices of neuroendocrine carcinoma of the lung may help to predict survival, 47 biopsies (from 37 males and 10 females; 16-82 years of age) were studied by light microscopy. Areal fractions of nuclei, cytoplasm, stroma, and blood vessels were determined using a standard point counting method. The counts were made in six non-coincident microscopic fields in each case, and the areal fractions of nuclei, of the entire tumour cell, stroma, blood vessels and the nuclear/cytoplasmic ratio were computed. In a multivariate linear regression analysis, survival in months after biopsy was considered the dependent variable of age and of all morphometric parameters listed above. The significance level was set at 5%. For all patients (disregarding staging) survival was negatively correlated (P < 0.001, multiple r = 0.5435) with age and nuclear/cytoplasmic ratio. When only patients with disease confined to the thorax (stages I, II and III) were taken into account, the accuracy of the function predicting survival increased considerably (P = 0.004, multiple r = 0.7957). The use of simple stereological methods, therefore, proved to be of value in predicting survival in patients with neuroendocrine carcinomas of the lung.