Knowledge of plant-weather relationships can improve crop management, resulting in higher quality and more stable crop yields. The annual timing of spring phenophases in mid-latitudes is largely a response to temperature, and reflects the thermal conditions of previous months. The effect of air temperature on the variability of hazelnut (Corylus avellana L.) phenophases (leafing, flowering) was investigated. Meteorological and phenological data for five cultivars were analysed over the periods 1969-1979 (P1) and 1994-2007 (P2) in Maribor, Slovenia. Phenological data series were correlated strongly to the temperature of the preceding months (R(2): 0.64-0.98) and better correlated to daily maximum and mean temperatures than to daily minimum temperatures. About 75% of phenophases displayed a tendency towards earlier appearance and a shorter flowering duration during P2, which could be explained by the significant temperature changes (+0.3°C/decade) from December to April between 1969 and 2007. An increase in air temperature of 1°C caused an acceleration in leafing by 2.5-3.9 days, with flowering showing higher sensitivity since a 1°C increase promoted male flowering by 7.0-8.8 days and female flowering by 6.3-8.9 days. The average rate of phenological change per degree of warming (days earlier per +1°C) did not differ significantly between P1 and P2. An estimation of chilling accumulation under field conditions during 1993-2009, between 1 November and 28 February, showed that all four of these months contributed approximately similar amounts of accumulated chilling units. The growing degree days (GDD) to flowering were calculated by an estimated base temperature of 2°C and 1 January as a starting date, given the most accurate calculations. In general, thermal requirements were greater in P2 than in P1, although this difference was not significant. Longer-time series data extended to other agricultural and wild plants would be helpful in tracking possible future changes in phenological responses to local climate.