A double-blind, placebo-controlled crossover study of the effect of sulphinpyrazone on blood viscosity and its determinants (haematocrit, plasma fibrinogen and plasma viscosity) was carried out in 7 healthy male volunteers. Apart from a transient rise in fibrinogen (p less than 0.05) after 3 days of sulphinpyrazone treatment, no significant change in viscosity factors was seen over a 14-day period.