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Phenological observations in olive tree (Olea europaea L.), on the first reproductive phase (budburst) and the winter chilling temperatures required for its onset, were analysed over a 4-year period (1998-2001). Research was carried out on two different cultivars growing in two Mediterranean olive-growing areas: `Ascolana' in central Italy and `Picudo' in southern Spain. The two main objectives of the study were: (1) to evaluate the different amounts of winter chilling, and their relationship with the budburst dates in outdoor olive plantations; and (2) to test the validity in the study areas of two chilling calculation methods, the Aron and the Utah method. Results show that for the Spanish cultivar the average chilling requirement of 997 h was approximately half of that recorded for the Italian cultivar (1848 h). Also, the year-to-year variability in chilling accumulation and the reproductive development date was observed to be higher in the Spanish area than in the Italian one, indicating a more consistent and predictable winter chilling response in the latter. As regards the validity of the methods, the Aron method seems to be more appropriate for use in warmer places, since it yielded better results in the Spanish site. [ABSTRACT FROM AUTHOR]