OBJECTIVES: Physical therapy is one important treatment option for patients with Low back pain (LBP). However, the appropriate timing of physical therapy referral remains unclear. This study aimed to assess the impact of timing of physical therapy referral on health care utilization and costs among patients with acute LBP.  METHODS: Patients between the ages of 18 and 64 years who had a new LBP consultation with a potential referring physician between January 1, 2009 and December 31, 2011 were identified, using Truven Health MarketScan® Database for the state of New York. They were grouped into cohorts based on the timing of physical therapy: Immediate Access Cohort (IAC)-received physical therapy within 3 days; Early Access Cohort (EAC)-received physical therapy between 4 days and 14 days; Delay Access Cohort (DAC)-received physical therapy from 15 days to 30 days; and Late Access Cohort (LAC)-received physical therapy between 31 days and 90 days. LBP-related health care utilization and costs occurring within one year after the index visit were recorded as outcome measures. RESULTS: A total of 4,945 patients were included with 1613 patients in the IAC cohort, 1593 in the EAC cohort, 1012 in the DAC cohort, and 727 in the LAC cohort. Multivariable generalized linear models controlling for patient characteristics showed that fewer patients in the IAC cohort used opioids, imaging, spinal injections, emergency room, and spinal surgeries. The average adjusted LBP-related medical and pharmaceutical costs incurred one year after the index visit was $3150.95 for the IAC cohort while those for the EAC, DAC, and LAC cohorts were $3327.33, $4475.24, and $6764.02, respectively. CONCLUSIONS: Timing of physical therapy is a significant factor related to health care utilization and costs among patients with acute LBP. When referral to physical therapy is warranted, early referral is associated with lower overall LBP-related health care utilization and costs.