Anecdotes from local residents and modeling studies suggest that deforestation may delay the onset of the rainy season (O) in western Brazil, but detection studies using climatological time series are not available. Here we investigate trends in O in the state of Rondonia, Brazil, a region that has been continuously deforested since the 1970s. Daily rainfall data from 16 station time series, spanning periods of at least 25 years, with five covering more than 30 years, are used. We define O as the first day after 1 September with rainfall greater than 20 mm d-1. A t test indicates that for stations that lie inside the major deforested area, O has significantly shifted to, on average, 11 days (and up to 18 days) later in the year over the last three decades. However, for stations that lie in areas that have not been heavily deforested, O has not shifted significantly. Nonparametric and parametric trend analyses all gave similar results for the change of O with time, and all of the statistically significant results indicated a delay in O. Twenty-five percent (four) of the stations analyzed showed a marked shift in timing of O: these stations are located inside deforested areas, primarily near the BR 364 highway that crosses Rondonia. Delayed onset may be a result of land use change, and this signal may strengthen in future: current delaying trends may be as great as 0.6 days per year, and after 30 years of deforestation the onset of the rainy season is expected to be 18 days later.