The Community Hospital Oncology Program (CHOP), funded under contracts by the National Cancer Institute (NCI) from 1981 to 1984, was designed to be a model for delivery of the most up-to-date cancer care in the community setting. Site specific patient management guidelines (PMGs) were developed by physicians who saw a majority of cancer patients in each community and represent a consensus of the most current information on pretreatment evaluation and management for each cancer site. There was the potential for PMGs to have a strong effect on physician practice patterns. A patterns of care study (POC) was conducted in the 17 CHOPs to determine the influence of guidelines on practice patterns. The practice elements examined in this report are clinical staging for breast and small-cell lung cancer, medical oncology consultation for breast cancer patients with positive lymph nodes, and radiation therapy consultation for rectal and small-cell lung cancer. These elements were in all CHOP guidelines. Except for physicians most active in the CHOP, the data provide no evidence of diffusion of guideline principles to the majority of practicing physicians. Although all guidelines contained discussion of the importance of staging before definitive treatment for breast cancer, only 33% of the 1,922 charts examined had a stage recorded. In small-cell lung cancer, 67% of the 388 charts examined had stage recorded. The years in practice of the primary physician had an inverse relationship to practice patterns as defined by the guidelines. Physician specialty was also a determinant of practice patterns for small-cell lung cancer. However, participation in guideline development and their dissemination did not significantly influence the patterns of care elements examined in this study.