Supply chains (SCs) operate in environments where there are various sources of uncertainties. This paper studies influences of fuzzy demand forecast on SC bullwhip effect. A two-echelon serial SC is considered and consists of production and inventory units. Uncertain customer demand and demand forecasted by the in-process facilities in the SC are described by imprecise linguistic expressions and modelled by fuzzy sets. Accuracy of demand forecast is measured via the level of uncertainty in demand. It is shown how fuzzy demand forecast affects the bullwhip effect in the SC, i.e., the sharper the fuzzy demand forecast, the smaller the bullwhip effect.