Finnish populations of the capercaillie Tetrao urogallus, black grouse Lyrurus tetrix and hazel grouse Tetrastes bonasia have been fluctuating in synchronous cycles. Mechanisms behind these fluctuations are not well known. Parasites may regulate their host population under certain conditions. The Finnish grouse harbour several species of intestinal parasites which may affect the populations. We studied the possibility of parasitic regulation in Finnish forest grouse using intestinal samples from hunted grouse collected in 1995–2000 and annual counts of grouse densities. We found that abundance and prevalence of the nematode Ascaridia compar affected negatively the grouse-community growth rate (all three species combined) and annual survival of grouse. In years of grouse density decline, the prevalence and abundance of A. compar was considerably higher than in other years. On the species level, this dynamic was most clearly seen in the capercaillie. The findings suggest the possibility of parasitic regulation by A. compar in the Finnish forest grouse community.