This paper investigates the effects of patient characteristics on frequency of referral at three group practices. Major findings are: for pediatricians, referral rate increases with patient age; among internists and general practitioners, variations in patient age, marital status, and payment source are consistently related to significant variations in referral rates. At the group with a large black clientele, blacks were referred significantly less than whites. A discussion of possible causes includes differences in medical conditions and utilization patterns as related to age and payment source, the economic burden of referral on fee-for-service patients, the social or psychological stresses on older unmarried patients, and variations by race in the patient-physician relationship. Methodological aspects of the study are presented in two technical appendices that deal with problems of estimation and tests of hypotheses used.