OBJECTIVE To determine whether there are seasonal variations in survival following out of hospital cardiopulmonary arrest.

DESIGN Prospective cohort study using the Heartstart (Scotland) database.

SETTING All of Scotland.

PATIENTS 10 890 people who suffered out of hospital cardiopulmonary arrest in the summer or winter between December 1988 and August 1997 inclusive.

INTERVENTION Univariate comparisons of 5406 arrests occurring in summer with 5484 in winter, in terms of patient characteristics, management, and survival using χ² and Mann-Whitney U tests. Multivariate analysis of the association between season and survival following adjustment for case mix.

MAIN OUTCOMES MEASURES Survival to discharge from hospital, survival pre-admission, in-hospital survival.

RESULTS Only 6% of people who arrested in winter survived to discharge, compared to 8% of those who arrested in summer (odds ratio 0.77, p < 0.001). People who arrested in winter had a poorer risk profile in that they were older, more likely to arrest at home, less likely to have a witness, and less likely to receive defibrillation. However, after adjustment for case mix, people who arrested in winter were still 19% less likely to survive compared to those who arrested in summer. Deaths pre-admission were significantly higher in winter (odds ratio 1.18, p < 0.05) but in-hospital deaths were not.

CONCLUSIONS People who suffer cardiopulmonary arrest in winter have a significantly lower likelihood of surviving. This is, in part, caused by the higher frequency of a number of recognised risk factors. However, their prognosis remains poorer even after adjustment for these factors.