The relative bioavailability of cyclosporin A (CsA) from a new microemulsion oral formulation (NEO) and the currently used soft gelatine capsule (SGC) was determined at steady state in 12 patients with rheumatoid arthritis. The AUC(0,12 h) values of cyclosporin A were significantly greater after NEO than SGC (2873 + 848 ng ml-' h (mean ± s.d.) vs 2355 ± 1128 ng ml-' h; P = 0.02, 95% CI (confidence interval of the difference: 81 to 955 ng ml-' h). Cmax values were significantly higher after NEO than after SGC (811 ± 244 ng ml-' vs 495 ± 291 ng ml-', P < 0.0001, 95% CI of the difference: 209 to 422 ng ml-').