A longitudinal field study conducted in Turku, Finland, traced the development of aggression as a function of the viewing of violence by children from the ages of 7 and 9 to the ages of 15 and 17 to explore the connection between violence viewing and viewers' aggression, and to shed light on the question of causality. The 220 subjects, both male and female, were interviewed six times during the years 1978 to 1985. The aggression of the subjects was measured with a modified version of the Peer Rating Index of Aggression, which asked each child to name all the children in his/her class who had displayed specific aggressive behaviors during the school year. Physical fighting was measured when the subjects were 15 and 17 years old using a procedure used by Patterson et al. (1984), and aggressive fantasizing was measured by six items from the fantasy scales derived by Rosenfeld et al. (1982). Subjects were also asked to estimate the amount of television viewing and video viewing they did and their cinema attendance. Data were analyzed using cross lagged panel correlations, process analysis, differences between the various subject groups, and path analysis. Positive, often statistically significant, correlations between violence viewing and aggression were found in all phases of this study. Detailed results of the data analyses are presented in four figures and six tables. (34 references) (CGD)