The authors examined whether there are differences in the determinants of physician use between aged and middle-aged individuals. Data were obtained through interviews and medical records from a probability sample of 460 individuals aged 45-89 years old. Physical health measures were important predictors for both age groups, but there were differences in the relative importance of specific variables. The number of diagnoses was more important in predicting physician use for the middle-aged and bed days was more significant for the aged. Psychologic factors played a slightly greater role in predicting physician use for the aged, and enabling and predisposing variables were somewhat more important for the middle-aged. Utilization during the year before the interview was associated with subsequent physician use for both age groups, but the correlation was weaker among the aged. Age differences in the predictors of physician use are believed to result from the unique illness response elicited by chronic, as opposed to acute, illness.