Objective To identify a seat pressure threshold that corresponds to signifi cant compression of the perineum. Design Quasiexperimental crossover. Setting Hospital radiology department. Participants Convenience sample of six male recreational cyclists. Intervention Compression of the corpus spongiosum and corpora cavernosa of the perineum without bicycle seat pressure (0%) and at 10%, 40% and 80% of the mean bicycling pressure was assessed using magnetic resonance imaging. Seat pressure was applied using a custom loading device. Outcome Measure Measure Diameter measurements of the cavernous spaces at the point of peak compression. Results The mean diameter values for the corpus spongiosum in the unloaded and 10% of load conditions were 105% and 172% greater than the 40% and 80% loaded conditions, respectively (p=0.002–0.004). The corpora cavernosa values displayed a similar trend. Conclusions Substantial compression of the perineal cavernous spaces may occur with total seat pressure values of less than 40% of typical seat loads.