The rationale for the present study was to evaluate the predictive role of Tc-99m-infliximab scintigraphy in therapy decision-making in patients with refractory monoarthritis and also candidates for intraarticular (IA) infliximab treatment. We studied 12 patients (5 with rheumatoid arthritis and 7 with spondyloarthropathy) with active monoarthritis (11 knees and 1 ankle) that had lasted for at least 3 months. Patients were evaluated clinically and ultrasonographically at baseline and 12 weeks after IA administration of infliximab. At the same time-points, Tc-99m-infliximab scintigraphy was performed: planar anterior and posterior images of arthritic joints were acquired at 6 and 20 h after injection and target-to-background (T/B) ratios were calculated. After treatment, a significant improvement in clinical and ultrasonographic parameters was recorded in six patients. Three patients had a partial response and three did not respond. Regarding scintigraphic evaluation, the T/B ratio analysis showed a significantly higher uptake in affected than in nonaffected joints before therapy (1.78 +/- 0.46 vs. 1.29 +/- 0.27, p = 0.006 at 6 h; 2.05 +/- 0.50 vs. 1.41 +/- 0.36 at 20 h, p = 0.002), and mean uptake at 20 h was also significantly higher than at 6 h (p = 0.0004). Scintigraphy showed a significant decrease in posttherapy T/B ratios of the affected joints (p = 0.0001 at 6 h and p = 0.0001 at 20 h), indicating a reduction in TNF into the affected joints. Most importantly, responders showed a significantly higher percentage increase in pretherapy uptake from 6 h to 20 h in the affected joints than nonresponders (p = 0.00001). The results of the present investigation suggest that Tc-99m-infliximab scintigraphy could be a useful tool to predict the clinical response to IA infliximab treatment in patients with refractory monoarthritis.