Fourteen young male volunteers measured their habitual dietary intake for 2 weeks, then were told to increase their dietary sucrose while decreasing their other carbohydrates for 2 weeks and finally told to revert to their habitual diet while continuing to record their intake. Measurement of plasma constituents revealed a significant fall in HDL-cholesterol concentration after the period on the high sucrose diet, and a return to the higher concentrations after resumption of the habitual diet. Twenty-six young men whose habitual diet contained more than an average quantity of sucrose followed a similar regime, except that they were told to reduce their sucrose for 2 weeks and to compensate by increasing the intake of other carbohydrates. In the event, unlike the volunteers in the first experiment, they were found not to have made the compensatory change when lowering their sucrose intake. Measurement of their plasma constituents showed that the reduction in dietary sucrose resulted in a significant fall in the mean concentration of triglycerides. There was no significant change in the mean concentration of HDL cholesterol, although there was an increase in the concentration in 11 of the 26 subjects.