Monitoring of diabetic foot infections is largely based on clinical assessment, which is limited by moderate reliability. We conducted a prospective study to explore monitoring of thermal asymmetry (difference between mean plantar temperature of the affected and unaffected foot) for the assessment of severity of diabetic foot infections. In patients with moderate or severe diabetic foot infections (International Working Group on the Diabetic Foot infection-grades 3 or 4) we measured thermal asymmetry with an advanced infrared thermography setup during the first 4–5 days of in-hospital treatment, in addition to clinical assessments and tests of serum inflammatory markers (white blood cell counts and C-reactive protein levels). We assessed the change in thermal asymmetry from baseline to final assessment, and investigated its association with infection-grades and serum inflammatory markers. In seven included patients, thermal asymmetry decreased from median 1.8°C (range: −0.6 to 8.4) at baseline to 1.5°C (range: −0.1 to 5.1) at final assessment (P = 0.515). In three patients who improved to infection-grade 2, thermal asymmetry at baseline (median 1.6°C (range: −0.6 to 1.6)) and final assessment (1.5°C (range: 0.4 to 5.1)) remained similar (P = 0.302). In four patients who did not improve to infection-grade 2, thermal asymmetry decreased from median 4.3°C (range: 1.8 to 8.4) to 1.9°C (range: −0.1 to 4.4; P = 0.221). No correlations were found between thermal asymmetry and infection-grades (r = −0.347; P = 0.445), CRP-levels (r = 0.321; P = 0.482) or WBC (r = −0.250; P = 0.589) during the first 4–5 days of hospitalization. Based on these explorative findings we suggest that infrared thermography is of no value for monitoring diabetic foot infections during in-hospital treatment.