The incidence, distribution density and cellular structure of argyrophil and argentaffin cells in 212 colorectal adenomas obtained from 128 colorectums mostly resected surgically were investigated. Out of the 212 adenomas, argyrophil cells and argentaffin cells were found in 162 (76.4%) and 128 (60.4%), respectively. Approximately 10 per cent of adenomas with silver-positive cells showed high distribution densities. There were 128 adenomas in which both argyrophil and argentaffin cells were found. In 33 out of the 128 Paneth's cells and goblet cells were also recognized. The incidence of argyrophil and argentaffin cells tended to become higher in adenomas of the distal colon and the rectum than of the proximal colon, apart from those of the ascending colon. Most of the adenomas with high distribution density of these cells were in the sigmoid colon and rectum, and the distribution pattern was somewhat similar to that of silver-positive cells in the non-neoplastic mucosa. There was no significant difference in the incidence of silver-positive cells among adenomas with different histologic types. Both silver-positive cells were usually found in the lower third portion of the adenomatous glands or tubules, the shape being flask-shaped or cylindrical, and they showed cellular atypicality as the adenoma cells did. It is speculated that silver-positive cells appearing within colorectal adenomas are substantially one of the structural elements of adenomas and are derived from immature adenomatous cells. The present study supports the unitarian theory that neuroendocrine cells in the gastrointestinal mucosa are of endodermal origin.