The rate of lateral spread of the agent(s) of hydropericardium syndrome was investigated in three controlled experimental trials, (1, 2 and 3), in which groups of 60, 50 and 70 birds had five, three and seven birds infected deliberately and were then maintained at stocking densities of 0.83, 1.00 and 0.71 square feet/bird. In each trial a control group of birds was reared at the same time. The non-parametric survival functions (Kaplan-Meier product limit) were fitted to the data and compared by the Mantel-Cox and Breslow tests. In trials 1, 2 and 3 the hydropericardium syndrome occurred with a median latent period of 9.5, 13.0 and 14.5 days, respectively, the median daily mortality rates were 3.4, 4.5 and 4.5 per cent and the cumulative mortalities were 40, 54 and 55.7 per cent. These results were not significantly different.