Pulmonary hypertension syndrome (PHS), or ascites, is characterized by elevated pulmonary arterial pressure and pulmonary vascular resistance accompanied by right ventricular hypertrophy (RVH) and fluid accumulation in the abdominal cavity. Experimental models are required for triggering PHS to study the pathogenesis of this syndrome and to select resistant genetic lines. Caffeine increases vascular resistance and promotes systemic hypertension in mammals, but a similar effect of caffeine on the pulmonary circulation had not previously been demonstrated. Two experiments were conducted to evaluate the impact of caffeine alone (Exp. 1) or in combination with cold temperature (Exp. 2) on parameters associated with PHS in young broiler chicks. In Exp. 1, 288 chicks were distributed among 24 pens and brooded at standard environmental temperatures, and on d 3 through 42 caffeine was added to the water at doses of 0 (control), 6.25, 12.5, 25, 50, and 100 mg/(kg BW·d). In Exp. 2, 192 chicks were distributed among 16 pens and brooded at cool environmental temperatures, and on d 3 through 42 caffeine was added to the water at doses of 0 (control), 15, 30, and 45 mg/(kg BW·d). In Exp. 1 caffeine administered at or above 12.5 mg/(kg BW·d) induced severe PHS and resulted in acute mortality and RVH ( < 0.05). Hematocrit also slightly increased by caffeine supplementation ( = 0.07). In Exp. 2 caffeine-treated broilers exposed to cold temperatures remarkably exhibited PHS incidences and developed RVH with right ventricular to total ventricular weight ratios of 30% or greater. Moreover, hematocrit significantly increased because of caffeine supplementation in cool ambient temperature ( = 0.002). Our data demonstrate that caffeine induces high incidences of PHS in broilers, which is exacerbated by exposure to low temperatures.