Informed participation in medical decisions is important because it demonstrates respect for the ethical principle of individual autonomy and increases the likelihood of reaching therapeutic goals. Twenty hospitalized patients were randomly selected and observed for six and a half hours to assess the degree to which informed participation was possible with diagnostic and therapeutic procedures. Resident physicians and patients were then interviewed about the rationale, benefit, risk, and alternative for each observed procedure. Commonly observed activities were injecting and giving oral medications, and performing invasive diagnostic procedures. Clinicians' communication involved rationale (43%) more often than benefits (34%), risks (14%), and alternatives (12%). Communication was similar when the procedures proposed were important and risky. Residents' and patients' interviews demonstrated limited congruence in shared understanding of rationale (57%), benefit (45%), risk (19%), and alternatives (25%). These results suggest that clinicians selectively impart information essential for informed patient participation, and highlight areas of clinician-patient communication in need of attention.