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This study investigated the causal effect of having a usual source of medical care on the timing of preventive services by women and children. Data on 17,110 children and 23,488 women from the 1988 and 1990 National Health Interview Surveys were used to estimate ordered probit models of the effect of having a usual source of medical care on the time since the last receipt of each preventive service (routine checkups for children; blood pressure checks, Pap smears, and breast exams for adult women; mammograms for older women). Two-stage instrumental variables methods were used to eliminate simultaneity bias. The existence of a usual source of medical care was strongly correlated with the earlier receipt of preventive services, and the relationship appears to be causal for Pap smears, breast exams, and mammograms. However, there was little evidence that having a …