Although São Paulo is one of the most dangerous cities in the world, very little is known about the variations of levels of crime in this Brazilian city over time. This article begins by investigating whether or not homicides are seasonal in São Paulo. Then, hypotheses based on the principles of routine activities theory are tested to evaluate the influence of weather and temporal variations on violent behaviour expressed as cases of homicides. Finally, the geography of space–time clusters of high homicide areas are assessed using Geographical Information System (GIS) and Kulldorff's scan test. The findings suggest that central and peripheral deprived areas show the highest number of killings over the year. Moreover, homicides take place when most people have time off: particularly during vacations (hot months of the year), evenings and weekends. Overall, the results show that temporal variables are far more powerful for explaining levels of homicide than weather covariates for the Brazilian case—a finding that lends weight to the suggested hypotheses derived from routine activity theory.