An analysis has been made of the relationship between neuron specific enolase (NSE) in serum and immunohistochemically identified occurrence of NSE in the primary tumour in 56 patients with small cell lung cancer (SCLC). Patients were referred to the Finsen Institute for treatment during a period of 18 months. Forty-six tumours (82%) were NSE positive. To compare this staining with the occurrence of NSE in serum, a histological staining index (HSI) was established by semiquantitative gradation of the staining. No significant differences were found between distribution of serum NSE values in different HSI categories, and a high ranking in HSI was not associated with a high level of serum NSE. Both univariate and multivariate analysis selected serum NSE and not HSI as the most influential prognostic factor in SCLC.