Obstetric anal sphincter injury (OASI) is associated with serious morbidity and reduced quality of life. The role of anorectal manometry (ARM) to guide treatment is unclear. We aimed to define the role of ARM and symptom assessment post OASI in predicting anal incontinence at long-term follow-up. Prospective evaluation of 205 consecutive post-OASI women who underwent baseline ARM and symptom assessment in a tertiary setting was undertaken. 99 were available for long term follow-up (median 6.6 years). Associations between post-OASI ARM testing and short- and long- term anal incontinence were examined in addition to clinical and obstetric factors. 4th degree tears and lower anal resting and squeeze pressures were associated with short-term anal incontinence. Females with anal incontinence at baseline, and females with lower anal resting pressure, were more likely to suffer from long-term fecal incontinence (FI). A best-fit cut-off value of 59 mmHg for baseline anal resting pressure predicted long-term FI, and none of the short-term asymptomatic females with an anal resting pressure of >59mmHg reported FI at long-term follow-up. Symptoms of anal incontinence and ARM in patients post OASI are both useful to predict subsequent long-term FI with a best-fit cut-off of 59mmHg for anal resting pressure. This provides rationale for routine ARM and accurate symptom assessment in all patients after OASI, to identify high risk groups to follow and treat, and it may assist decision-making regarding mode of subsequent obstetric deliveries.