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Purpose: To determine the diagnostic efficacy of diffusion-weighted magnetic resonance imaging (DWI) in the diagnosis of perianal fistula and its complications. Material and methods: This is a retrospective study based on the data of 47 patients with a clinical diagnosis of perianal fistula, who had an MRI study performed on a 1.5-T GE Signa MR scanner. DWI sequences were done using 3 different b-values. Other routine MR sequences were included. The MR images were studied to compare the diagnostic efficacy of the DW MRI sequence and other sequences in diagnosing perianal fistula and its complications. Apparent diffusion coefficient (ADC) values of abscesses and inflammatory soft tissue lesions were measured using ADC maps. The standard reference to obtain diagnostic efficacy was post-surgical data. Results: Seventy-nine perianal fistulas were diagnosed in 47 patients who had undergone an MRI study. The sensitivity and specificity of different MR sequences in diagnosing perianal fistulas are T2 FSFSE: 92% sensitivity; DWI: 96% sensitivity; combined T2+DWI: 100% sensitivity; and post-gadolinium T1 FS has 100% sensitivity in diagnosing perianal fistulas. The mean apparent diffusion coefficient for the abscess in our study was 0.990 ± 0.05 × 10-3, and the mean apparent diffusion coefficient for an inflammatory soft tissue lesion was 1.440 ± 0.05 × 10-3. The optimal ADC cut-off for the abscess was 1.098 × 10-3 mm2/s showing 100% sensitivity and 93.8% specificity. Conclusions: DW imaging is a reliable sequence to diagnose perianal fistula and its complications. Measurement of ADC values is reliable in diagnosing perianal abscess collection. DWI sequence helps patients with renal impairment in whom IV gadolinium is contraindicated.