In this study we aimed at evaluating the modifications in the pharmacokinetic profile of cyclosporin A (CyA) after conversion from standard formulation (CyA-ST) to a new formulation (CyA-NF, Sandimmun Neoral) in patients with rheumatoid arthritis (RA). It was an open, crossover study that involved 15 RA patients who were on stabilized treatment with CyA-ST. The patient continued receiving CyA-ST (mean dose of 3.0 +/- 0.7 mg/kg per day) for 3 weeks and then converted 1:1 to CyA-NF for a further 3 weeks. CyA pharmacokinetics were established on day 1 (CyA-ST evaluation) and +21 (CyA-NF evaluation). The results showed that the bioavailability of CyA-NF was greater than that of CyA-ST (AUC tau, bss: 3335 +/- 1300 vs 2667 +/- 1155 ng.h/ml, P = 0.0073; AUC tau, bss ratio 1.26 +/- 0.40 vs 1.0 as reference, P < 0.05), with higher and earlier peak blood concentrations (Cmax: 677 +/- 256 vs 475 +/- 213 ng/ml, P = 0.0329; tmax: 1.5 +/- 0.7 vs 2.6 +/- 1.6 h, P = 0.0720). The pharmacokinetic profile of CyA-NF showed greater between-patient reproducibility (lower CV% for all of the considered parameters). In conclusion, when using CyA-NF instead of CyA-ST, greater and more constant exposure to CyA should be expected.