Abstract: Stem rot (Sclerotinia sclerotiorum) has increased in importance in England recently, but risk assessment and decisions on fungicide use remain difficult for individual crops. Stem rot levels in winter oilseed rape in 2007 were the highest yet recorded and the most severe since 1991. This prompted an investigation of the factors affecting the development of the 2007 epidemic. Petal tests indicated that inoculum was present despite very low rainfall in the spring. Hourly weather data for the flowering periods in 2006 (a ‘non-epidemic’ year) and 2007 were collated from a range of metereological stations to represent the main areas of production. The number of periods during flowering that met the infection criteria used in SkleroPro (minimum of 23 h with relative humidity > 80% and temperature > 7 °C) were identified. Further comparisons of infection events were made between ADAS Rosemaund, Hereford (west) and ADAS Boxworth, Cambridge (east) for the years 1991-2007. The SkleroPro infection criteria were useful for identification of infection events and interpretation of fungicide experiments where sclerotinia inoculum was present. At ADAS Boxworth where little sclerotina developed in most years, inoculum was considered to be limiting. The development of more effective guidance for stem rot management will require quantification of inoculum and prediction of infection events.