As decompression sickness (DCS) may occur unexpectedly after 'safe' dives it was hypothesised that the weather and tidal factors could contribute to the risk. One hundred and seventy seven cases of DCS were identified from the Institute of Naval Medicine's diving accident records and allocated to a 'safe' group or control 'risky' group, depending on the dive profiles. Comparison of the prevailing environmental conditions between groups revealed significant differences in air temperature and windchill (p = < 0.001 for all dives) and for air minus water temperature (p = < 0.01 for all dives). The results imply that exposure to a cold thermal environment following diving, particularly when the air temperature is colder than the water temperature, may be a previously unrecognised risk factor for DCS.