Three hundred and sixty seven neoplastic colorectal polyps removed at endoscopy of which 34 were complicated by invasive carcinoma, were reviewed clinically and histologically to assess the prevalence and clinical significance of venous and or lymphatic invasion. Traditional stains for histological assessment were supplemented by immunohistochemical methods. Venous and or lymphatic invasion was present in six cases (17.6%), with a statistically significant association with recurrent carcinoma or Dukes's C carcinoma in polyps otherwise regarded as completely excised. The presence or absence of venous and or lymphatic invasion in malignant polyps should be documented as if present, further treatment is indicated. A combination of haematoxylin and eosin and elastic-van Gieson stains will usually identify the presence of vascular invasion. Where a discrepancy arises, however, additional immunohistochemical stains may be of value.