Objective. Mannose binding lectin (MBL), a member of the collectin family proteins, is a major molecule of the innate immune system; MBL deficiency is associated with increased susceptibility to infections. As gastrointestinal and genitourinary infections are suggested to be among the etiological factors of spondyloarthropathies (SpA), we investigated MBL deficiency in ankylosing spondylitis (AS) and undifferentiated SpA (uSpA). Methods. One hundred seven patients with AS, 43 patients with uSpA, and 74 healthy controls were studied. Disease activity, radiological scores, and demographic features were recorded. MBL levels were measured with standard ELISA kits. Results. Median MBL levels in AS, uSpA, and controls were 2705 (range 0-5861) ng/ml, 2897 (36-7586) ng/ml, and 3468 (0-7950) ng/ml, respectively. No significant differences were observed in median MBL levels and the prevalence of MBL deficiency between the groups. Bath AS Radiological Index scores were not affected by MBL levels. However, although statistically not significant, radiographic damage quantified by modified Stoke AS Spine Score (mSASSS) was 3 times higher in AS patients with MBL deficiency. Disease activity, clinical picture, and therapies were not associated with MBL levels. Conclusion. In AS patients with MBL deficiency, there was a tendency towards a more severe radiographic progression detected by mSASSS.