The influence of annual climate variations on the performance of solar thermal collectors in the northern part of Europe has been investigated. The annual solar collector energy output has been calculated with the MINSUN simulation program using hourly, measured climatic data for the years 1983-98 for three cities situated in the south (Lund), central (Stockholm) and north (Lulea) of Sweden. A synthetic year created with the Meteonorm weather simulation program was also used in the simulations. Two solar thermal collectors were modelled: a flat plate solar collector and a tubular vacuum collector, both of commercial standard. The thermal energy output is strongly correlated to the annual global irradiation at a horizontal surface. The annual average energy delivered from the flat plate collector was 337 kWh/m(2) for Stockholm (337 for Lund and 298 for Lulea), and from the vacuum tube collector 668 kWh/m(2) for Stockholm (675 for Lund and 631 for Lulea) at an operating temperature of T=50 degreesC. Maximum deviations from the average value for this 16-year period are around 20% for the flat plate and 15% for the vacuum tube collector, at T=50 degreesC. The relation between global irradiation on a horizontal surface and the annually collected thermal energy at a constant operating temperature could be fitted to a linear equation: q(u)=aG(0 degrees)+bT, where q(u) is the energy output from the collector, G(0 degrees) the global irradiation at a horizontal surface, T the average temperature of the collector fluid, and a and b fitting parameters in a double linear regression analysis.