A relationship between certain weather conditions and the frequency of some diseases has been recognised for a long time. During a period of two and a quarter years, the course of infusion urograms in 6,862 patients was carefully monitored. At the same time, records were kept of the weather, including the general weather situation, changes in the weather, its dynamics, atmospheric pressure and "vorticity". 7.96% of patients showed side effects from the contrast medium. 32.6% of all patients admitted to having been affected by the weather, with an incidence of side effects of 9.7%. Older patients and over-weight patients tended to suffer fewer side effects. There was no correlation with the seasons. As regards the general weather situation, there was an increased incidence associated with changes coming from the east, fewer incidents during the in-flow of cold air and high pressure situations. There were marked differences during periods of high and low pressure ("vorticity"). High pressure situations and periods of increasing barometric pressure had complication rates of 5.3% and 4.7% respectively, low pressure, 11%. Statistical analysis showed no significant difference between the groups, but tendencies were unmistakable. A large number of individual factors are known to have an effect, such as abnormalities of the complement system, antigen-antibody reactions and others, but the importance of each factor remains uncertain. The present results indicate that the reaction of a patient at the moment of the investigation also plays a significant part.