We studied the incidence, clinicopathological relevance and prognostic significance of serotonin immunoreactive cells in a clinically well-documented series of 300 large bowel cancer patients. Serotonin immunoreactive cells were detected in 8 per cent of the carcinomas, occurring either as focal clusters (4.7 per cent) or as occasional single cells (3.3 per cent). Both types frequently displayed presence of mucin and/or immunoreactivity for secretory component as well. In respect of localization, stage or tumour extension and histological grade tumours with serotonin immunoreactivity showed no significant differences in comparison with carcinomas lacking this feature. In terms of survival, however, the tumours with serotonin immunoreactivity demonstrated a more aggressive clinical course in comparison with tumours without these cells. This phenomenon reached borderline statistical significance. It is therefore concluded that the study of serotonin immunoreactivity in large bowel carcinomas enables the identification of a subpopulation of colorectal carcinomas with a relatively poor prognosis.