Background: Accurate diagnosis of a mullerian anomaly is essential for appropriate management and prevention of complications. However, diagnosis is often missed or delayed. Case: This is a case of a nulliparous woman with a mullerian anomaly diagnosed at the age of 36 despite prior evaluation with ultrasound and laparoscopy. Magnetic resonance imaging (MRI) suggested a unicornuate uterus with a right non-communicating rudimentary horn. Hysteroscopy and chromopertubation confirmed the diagnosis. The rudimentary horn was resected laparoscopically using bipolar energy. Conclusion: An undiagnosed mullerian anomaly should be considered in the setting of persistent dysmenorrhea. Three-dimensional ultrasound or MRI should be used judiciously in patients with refractory dysmenorrhea or when ultrasound raises concern for a mullerian anomaly.