We investigated the relationship between atrial fibrillation and the red cell parameters haematocrit, red cell concentration and mean corpuscular volume in 95 female patients with mitral stenosis (mean age 51+/-11 years, 38 patients in atrial fibrillation, 57 patients in sinus rhythm) who had undergone full blood examination and right and left heart catheterisation. Haematocrit was a positive correlate of atrial fibrillation (r=0.29, p<0.009) and a negative correlate of cardiac index (r=-0.37, p<0.003), but cardiac index was the only independent correlate of haematocrit on multivariate analysis (r2=0.14). The mean corpuscular volume was a positive correlate of age (r=0.42, p<0.0001) and atrial fibrillation (r=0.29, p<0.005) and a negative correlate of cardiac index (r=-0.22, p<0.04) and red cell concentration (r=-0.56, p<0.0001). On multivariate analysis, however, only age and red cell concentration were independent correlates of mean corpuscular volume (r2=0.43). Cardiac index was inversely correlated with both haematocrit and red cell concentration in the subgroup of patients with atrial fibrillation but not those with sinus rhythm. This study demonstrates that the major determinant of the higher haematocrit in mitral stenosis patients with atrial fibrillation is an associated reduction in cardiac index.