Background: Lumbar bone stress injuries (LBSI) are a problem in cricket, especially for youth fast bowlers, given their high incidence and prolonged recovery and negative career consequences. Aim: The BAILS (Bowling workload And Investigation of Lumbar stress injuries) study investigated the incidence, and risk factors for LBSI, including seasonal changes in female and male youth community cricket bowlers. Methods: A prospective, observational study of young cricket bowlers was conducted during the 2023/24 cricket season in southeast Queensland. Measures included: anthropometrics, previous LBSI, other bone stress injury, bowling arm dominance, bowler type, pre-season bowling workloads; sleep behaviours (adolescent sleep hygiene scale); calcium intake (AusCal questionnaire); other bone-relevant physical activity (BPAQ); bone parameters from ultrasound (QUS, REMS), back extensor strength, hip and pelvic stability (star excursion balance test), and maximum bowling speed (Stalker II radar gun). Participants recorded daily balls bowled, intensity of each bowling session, and duration of other physical activity in a logbook through the season to monitor workload. Workload was expressed as: 7, 28, and 90 day- volume of balls, acute:chronic workload ratio (ACWR), number of sessions and consecutive sessions, and volume of other activity. Participants were monitored for low back pain. Symptomatic bowlers were assessed by a Sports Physician and referred for MRI. LBSIs were classified by bone stress severity and fracture type. Results: 102 community cricket bowlers (M=78, F=24) of mixed ability participated in pre-season testing and were monitored for LBSI. Sixteen participants had a previous LBSI (M=13, F=3), 4 within the past 3 months. During season 2023/24 there were 16 new LBSI's (16:100 injuries per season), 13 male (17:100 injuries per season), and 3 female (13:100 injuries per season), and 5 recurrent injuries (48:100 injuries per season). Overall incidence was 32 injuries per 100 players (M=26, F=6) in 26 bowlers. Most LBSI were at L4 (31%) and L5 (35%) and were on the side contralateral to bowling arm (81%). Severity of LBSI was mostly incomplete stress fracture with bone marrow oedema (65%). Compared with the uninjured bowlers, injured bowlers were 1.3 years more past peak height velocity (yPHV) (95% CI 0.1-2.4), 10.5 cm taller (95% CI 1.3-14.5); 10.1 kg heavier (95% CI 0.7-19.0); bowled 10.6 km/hr faster (95% CI 1.7-19.5); had 0.024 g/cm2 higher lumbar spine BMD (95% CI 0.003-0.052), 0.069 g/cm2 higher femoral neck BMD (95% CI 0.033-0.113), and 8.2 Db/MHz higher front foot calcaneal BUA (95% CI 0.5-15.9); but -0.3 lower adolescent sleep hygiene score (95% CI -0.5- -0.03). After adjustment for years from age of, only past history of an LBSI was significant as a risk factor for an in-season LBSI (p=0.029). Injured fast bowlers had bowling workload patterns with large magnitude of peak ACWRs of 2.4 (28 day) and 6.7 (90 day), but these were also high in uninjured bowlers 2.8 (28 day) and 5.8 (90 day). Injured bowlers had fewer consecutive rest days in the 7 days prior to injury (p=0.015) and did more other activity in the 28 days prior to injury (p=0.015). Conclusion: LBSI incidence is high in female and male youth cricket bowlers. A mid-aged teenager who is 2-3 years post aPHV and is taller, heavier and bowls faster with sudden spikes in workload was most likely to develop an LBSI, irrespective of potentially stronger bones.