The effect of diacetylrhein on lymphocyte membrane fluidity in osteoarthritis patients before and after 10 and 30 days of treatment was studied using the 1,6-diphenyl-1,3,5-hexatriene fluorescence polarization. Moreover we studied the in vitro effect of rhein, the active metabolite of diacetylrhein, on lymphocyte membrane fluidity of controls using the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and its cationic derivative 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene. Our results showed that the patients with active osteoarthritis have higher fluorescence polarization values than those in the other osteoarthritis patients and in the controls. Moreover after 10 days of diacetylrhein treatment, we observed a significant decrease of fluorescence polarization values only in lymphocytes of the patients who showed active osteoarthritis before therapy. However after 30 days of treatment, significant changes of fluorescence polarization values were observed also in those patients who did not show painful osteoarthritis before diacetylrhein treatment. We observed that in vitro rhein also induces an increase in lymphocyte membrane fluidity, more evident with 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene with respect to 1,6-diphenyl-1,3,5-hexatriene.