Literature suggests advantages for co-locating behavioral health care in primary care. We compared the impact of location of services on attendance at behavioral health appointments when access to care was assured for externalizing behavior problems with referral as usual. Two primary care pediatric practices had an evidence-based parenting program co-located in the practice for parents of children aged 2-12 years and two practices had the program available using an enhanced-referral procedure for locations external to the practices. The program was available at the regional children's hospital (referral as usual). During an 8-month period, the rate of attendance at first appointments was significantly higher in the co-located than the enhanced referral condition (.38 and .12 % of patient visits, respectively; χ(2) = 13.32; p < .0003; OR = 3.10; 95 % CI: 1.63, 5.89). These outcomes, while low, were better than the near 0 rate of attendance to referral as usual. Availability of behavioral health services in both conditions increased rates of attended appointments. However, the low rates of attendance indicate increasing availability of services, alone, is not sufficient to decrease the unmet need of children with behavioral problems. Factors other than availability must be addressed in order to improve outcomes for children.