Temperature series from Stockholm and Uppsala in southern Sweden indicate that summers from the mid-18th century until around 1860 were, on average, warmer than the 1961-90 mean. The station histories suggest that the early observations could have been positively biased, for example because of insufficient radiation protection. We investigate if independent support for warm summers in the early period can be obtained from other climate variables. Using stepwise multiple regression analysis we investigate nine potential predictor variables: six air circulation indices, precipitation, air pressure and cloud amount. Three of these variables - cloud amount (the most important one), meridional geostrophic wind, and air pressure - together explain 65% of the June-August temperature variance in the calibration period 1873-2000. Application of the regression relationship back to 1780 shows that the model is equally successful in predicting year-to-year temperature variability before 1873 as it is in the calibration period, whereas the low-frequency component is poorly reconstructed in the early period. This reduced skill is primarily due to poorer data quality of the predictor variables in the early period, in particular the cloud amount series. The observed decadal mean temperatures during 1780-1860 are found to be above the upper limit of a 95% confidence interval that accounts for uncertainties both in the regression relationship and in the cloud amount series. We conclude that the observed temperatures before around 1860 are, therefore, most likely positively biased. The size of this bias cannot be accurately determined from the evidence used here, but seems to be about 0.7-0.8°C for both stations. A comparison with long instrumental temperature series from central Europe suggests a slightly smaller bias (0.5-0.6°C). For more accurate assessment of the Stockholm and Uppsala temperatures, we recommend that extensive homogeneity testing of other long northern European temperature series are undertaken.