We conducted this study to determine if serum galactomannan (GM) can be used as a marker to implicate the invasiveness of allergic fungal rhinosinusitis (AFRS), and correlate this value with the aggressiveness of disease documented via computed tomography (CT). All paranasal CT scans done for AFRS patients prospectively over a five-year period (2015–2019) were included. An indigenous 20-point score was used to document the extent of bone erosion seen on CT, wherein a higher score meant a greater extent of bone erosion. It was then correlated with serum GM scores. The median CT scores of galactomannan-positive (GM+) patients were compared with the median CT scores of galactomannan-negative (GM−) patients 3 using Mann–Whitney U test. The patients were divided into five groups based on the extent of disease-No bone erosion, erosion of only sinus wall/orbit, 3 erosion of orbit and skull base, erosion of only skull base and lateral extension of disease into infratemporal fossa (ITF). Subgroup analysis was conducted over mean GM values in these groups using ANOVA test. p-value < 0.05 was considered significant. Statistical analysis was performed using SPSS version 25.0. A total of 92 patients were included (56 males, 36 females). No statistically significant difference was found (p-value = 0.42) between the CT scores of galactomannan-positive (GM+) group and galactomannan-negative (GM−) group. The mean GM scores amongst the five sub-groups did not show a statistically significant difference. Serum galactomannan values correlate poorly with aggressiveness of disease quantified on non-contrast CT of paranasal sinuses.