In order to study the effect of high population density on the condition, blood characteristics and helminth parasitism of mountain hares (Lepus timidus), 12 specimens were shot in December 1982 and 12 more in February 1983 on the west coast of central Finland (group 1, dense population). In addition 14 hares were shot in December 1982 about 100 km from group 1 (group 2, dense population). Group 3 consists of 15 hares from stable, rather low density populations shot in southern Finland during three previous winters. The hares in group 1 were the lightest, had the least fat and were the most seriously infected with Protostrongylus pulmonalis and Trichostrongylus retortaeformis, while those in group 2 were the heaviest and had the highest Ca, Mg, alkaline phosphatase and creatinine values. The group 3 hares had the most fat. The group 1 animals shot in February 1983 had higher Ca, Mg, triglyceride and cholesterol values than those shot in December 1982. It seems that high population density combined with a lack of suitable food leads to poor condition and high endoparasite abundances. The differences in Ca and Mg are probably due to diet. The higher creatinine values in group 2 and in the hares with little or no T. retortaeformis infection may be due to the greater muscle mass.