Aim: We aimed to investigate the correlation of acute flank pain incidence with number of pregnancies and hydronephrosis. Material and method: Forty-eight patients admitted with acute flank pain have been included in this study. Patients with urinary tract infection, abnormal urinalysis, kidney or ureteric stone have been excluded. Twenty-four were nulliparous and the remaining pregnancies were multiparous. All patients had urinary ultrasound (US) by the same radiologist. Visual analogue scale (VAS) was used. All the patients had conservative management firstly. same radiologist. Visual analogue scale (VAS) was used when patients admitted to the hospital. All the patients had intravenous fluid therapy as conservative management, paracetamol as analgesic treatment and oral nitrofurantoin as antibiotherapy. Patients who did not benefit from conservative treatment had ureteral JJ stent placement. Results: Nulliparous pregnant were younger and had earlier gestational weeks (25.1±3.7 – 28.7±3.8 p=0.004 and 22,9±3.7 – 26.3±4.0 p=0.005), but higher VAS scores. In the nulliparous pregnant women, a significant, medium level positive correlation was found between the degree of hydronephrosis and the VAS scores. It was observed that the higher the hydronephrosis, the higher the VAS scores were. On the other hand, in multiparous pregnant women, no correlation was detected between the degree of hydronephrosis and the VAS scores. In comparison of two groups in relation to BM, hydronephrosis degree, hydronephrosis side, serum creatinine levels, type and side of pain, no significant difference was identified. Forty-two percent of patients described a colic pain. While 91.7% of the patients benefitted from conservative treatment, 4 patients had JJ stent insertion. Conclusion: This study demonstrates that there is a correlation between acute flank pain and hydronephrosis and that the degree of hydronephrosis increases as the pain intensifies, especially in the nulliparous, such correlation is stronger. Keywords: Pregnancy, hydronephrosis, acute flank pain, nulliparous, multiparous