A study was carried out in 40 patients undergoing minor surgery to investigate the effect of hydroxychloroquine on haemorrheological parameters which might explain its reported anti-thrombotic action. Twenty patients received 600 mg to 1400 mg hydroxychloroquine sulphate in the 48-hour period before operation and the control group of 20 patients received placebo. Blood samples were taken during the 24 hours before and on the day after operation. The results showed that in the hydroxychloroquine-treated group the transoperative increase in fibrinogen was associated with a reduction in plasma viscosity compared with an increase in the control group. Furthermore, the mean value for whole blood viscosity at middle shear rate decreased post-operatively in the drug-treated group whereas it increased in patients who had received placebo.