The present study was undertaken to analyse the extent of apoptosis in operated small cell lung carcinoma (SCLC) by using in situ labelling of the oligonucleosomal DNA fragments by terminal transferase. The extent of apoptosis was compared with the cell proliferation activity, as determined by Ki-67 immunohistochemistry; with the volume density of necrosis (per cent), as determined by the morphometric point counting method; and with the occurrence of immunohistochemically detectable p53 and bcl-2 proteins. By in situ labelling, remarkably high apoptotic indices (from 0.08 to 8.10 per cent) were seen in SCLC. A high percentage of SCLSs also showed an exceptionally high proliferation activity. Aberrant accumulation of p53 protein was seen in 37.5 per cent and bel-2 overexpression in 50 per cent of SCLCs. Necrosis was seen in 82.5 per cent of SCLCs. The extent of apoptosis was inversely related to the extent of tumour necrosis (P = 0.05) and to p53 protein accumulation (P = 0.008). A positive association was found between the extent of apoptosis and bel-2 immunoreactivity (P = 0.02). The apoptotic indices (per cent) correlated with the age (P < 0.05) and total smoking time of the patients (P = 0.06).