A series of 46 primary bronchogenic carcinomas for which thymidine labelling index (%TLI) (in all cases) and tumour doubling time (DTact) (in 13 cases) had previously been measured were followed up for 5 years and these data compared with length of post operative survival, tumour volume at operation and pathological staging. We found no correlation between reduced survival and higher tumour %TLI, indeed the reverse may be true. Larger tumours tended to have higher labelling indices considering either primary tumour volume or 'T'-category. Five year survivors had smaller tumours, tended to have T1 tumours and Stage I disease but did not have significantly lower tumour %TLIs. No relationship was found between DTact and any other parameter.