A three-dimensional system for long-term culture of human colorectal adenomas. Studies of the adenoma-carcinoma sequence in the colon and rectum have been limited by the paucity of experimental models of adenoma growth and progression. Progress recently was reported in the development of monolayer culture systems. The principal objective of this study was to develop a primary culture system for colorectal adenomas that would simulate three-dimensional in vivo growth. We used a calcium alginate encapsulation technique that was previously described for established tumor cell lines. Briefly, fresh resected specimens were washed, minced into small multicellular particles called microadenomas, and encapsulated in 1% calcium alginate pellets. The pellets were maintained in minimum essential medium containing 10% fetal bovine serum at 37 degrees C in humidified atmosphere of 95% air, 5% CO2. Ten of eleven adenomas, including six tubular, three tubulovillous, and one villous have been successfully cultured for 34 to 162 days. Cell viability was confirmed histologically by light and electron microscopy. The cells were characterized as epithelial by morphologic features and ultrastructural studies, which showed a high degree of cellular differentiation, including villous brush borders and many desmosomes. Both tubular and villuslike structures have been observed in vitro, correlating in some cases with the histology of the parent adenoma. Measurements of proliferative activity by [3H]thymidine autoradiography or immunohistochemical staining with the monoclonal antibody Ki-67 demonstrated growth fractions of 9% to 25%. A simple, highly efficient primary culture system was developed for the long-term maintenance of adenomas that promotes three-dimensional growth patterns and growth rates analogous to those seen in vivo. This model provides an opportunity to develop an experimental system for longitudinal studies of pathologic and molecular parameters in adenoma progression to carcinoma. 