Background: This study aimed to evaluate the predictive values and relationships of serum prostate-specific antigen (PSA) levels, delta neutrophil index (DNI), neutrophil-lymphocyte ratio (NLR), and other hematological parameters in patients diagnosed with acute prostatitis. Methods: Serum PSA levels and hemogram parameters of patients diagnosed with acute prostatitis were retrospectively analyzed. Healthy patients of the same age group were assigned to the control group. WBC, neutrophil, lymphocyte, thrombocyte (PLT), mean platelet volume (MPV), plateletcrit (PCT), NLR, and DNI were determined. Serum total PSA and C-reactive protein (CRP) levels were also assessed. Results: Total PSA levels (3.48 [1.11–6.66]) in the acute prostatitis (AP) group were significantly higher than those of healthy men (0.82 [0.47–1.39]) ( p < 0.001). Total CRP levels (3.88 [1.50–22.03]) in the AP group were significantly higher than those in healthy men (1.15 [0.89–2.00]) ( p < 0.001). The sensitivity and specificity of PSA at a cutoff value of 1.52 were 68.4% and 79.7%, respectively. The NLR value in the AP group was 2.62 (1.87–4.42), compared to 1.63 (1.18–2.20) in the healthy group ( p < 0.001). Differences in WBC, neutrophil, and lymphocyte counts were also statistically significant ( p < 0.001). There were no significant differences in PLT, PCT, or DNI values. Significant positive correlations were observed between PSA, CRP, and DNI values (all p < 0.001). Conclusion: This study demonstrated the usefulness of NLR, PSA, CRP, and WBC as predictors of acute prostatitis. NLR is a simple, inexpensive inflammation parameter that correlates well with CRP levels.