Introduction: Cystic hygroma is a type of cyst that occurs when lymphatic fluid collects in the lymph nodes which occurs in infants and can cause significant swelling in the neck or other parts of the body. Sildenafil is a possible medication that can be used to treat erectile dysfunction in infant. Aim of the study: The aim of this study was to assess the effectiveness of oral sildenafil in the treatment of cystic hygroma in infant. Methods: This retrospective study was conducted from January 2020 to June 2022 in The Division of Pediatric Surgery, Bangladesh Shishu Hospital & Institute and three other private clinics at Dhaka. Total 55 patients (aged between 2 months to 1 year) with cystic hygroma were included in the study. Result: In the present study, the median (Range) age of 55 patients was 6.2 (2-12) months. The median (Range) weight of 55 patients was 6.5 (4-12) kg. In this study, majority (60%) of the patients were male. More than half (52.73%) patients had the lesion on neck. The median (Range) pretreatment size of lesion was 13.30 (9.88-16.93) cm2. The median pretreatment size of lesion was 13.30 cm2 and median post treatment size of lesion was 0.00 cm2. There was highly significant difference (p <0.001) between pre-treatment and post-treatment size. In this study, excellent response was found in 69.09% patients, good response was found in 21.82% patients. In the current study, 3 patients (5.45%) had cough, 4 patients (7.27%) had fever, 2 patients (3.64%) had rhinorrhoea and 3 patients (5.45%) had vomiting. In the current study, 12 (21.82%) patients had adverse effects. Cost of sildenafil per tablet was 34 cents (100 cents =1 $). Conclusion: Oral sildenafil resulted excellent size regression and low adverse effects for the treatment of cystic hygroma. It was also very cost effective and convenient to administer.