Stable oxygen isotope analyses at annual, 2-, 5-, 10- and 20-varve sample resolutions were carried out on two selected varve intervals from the interglacial sediment record of the Pianico palaeolake. These sediments are particularly suitable for ultra-high-resolution isotope analyses on lacustrine endogenic calcite because of the exceptionally well-preserved varve structure. A bias through detrital contamination can be excluded because microscopically controlled sampling enabled selecting detritus-free samples. The studied sediment intervals comprise 352 and 88 continuous varve series formed during periods of rapid climate change at the onset and end of a marked millennial-scale cool interval during the Pianico Interglacial. The most intriguing result is a pronounced short-term oscillation in the bi-annually resolved isotope record superimposed on the general decreasing and increasing delta-18O trends at the climatic transitions that is recorded at lower sample resolution. Spectral analyses of the bi-annual time series reveal periodicities indicating solar and NAO controls on the delta-18O record. Multiple delta-18O measurements from endogenic calcite of individual varves showed variations of up to 0.6 per mill, thus larger than the observed inter-annual variability and most likely explained by seasonal effects.