Hyperemesis gravidarum(HEG) is one of the severe health problems in early pregnancy. Obstetricians should be aware of systemic inflammation in HEG patients to provide better preventive strategies. Hyperemesis gravidarum(HEG) is one of the most common causes of hospitalization in early pregnancy. Complete blood count parameters can be used as inflammatory markers in patients with HEG. We aimed to investigate the Systemic Immune-Inflammation Index (SII)in predicting the severity of HEG. This cross-sectional study was performed with 469 pregnant women diagnosed and hospitalized with HEG. The study parameters were calculated from complete blood count tests and urine analysis. Demographic characteristics, the Pregnancy Unique Quantification of Emesis (PUQE) scale values, and ketonuria levels at hospital admission were recorded. The neutrophil‑to‑lymphocyte ratio (NLR), platelet‑to‑lymphocyte ratio (PLR), lymphocyte-to-monocyte ratio (LMR), and SII, calculated by the formula (neutrophil × platelet/lymphocyte), were evaluated for predicting the severity of HEG. There was a positive correlation between the increased degree of ketonuria and SII. The cut-off value of SII for predicting the severity of HEG was 1071.8 (AUC 0.637, 95% CI (0.582-0.693), p<.001), and sensitivity and specificity were 59% and 59%, respectively. The cut-off value of SII to predict the length of hospitalization was 1073.6(AUC: 0.565, 95% CI: (0.501-0.628), p= 0.039); sensitivity and specificity were 56.3% and 55.5%, respectively CONCLUSIONS: : The clinical utility of SII in predicting HEG severity is limited due to relatively low sensitivity and specificity. Further research is needed to determine the importance of inflammatory indices in HEG patients.