It is unknown whether physicians treating critically ill patients have realistic perceptions of their patients' prognoses. We sent a survey by email to Finnish anesthesiologists to investigate their ability to estimate the probability of 1-year survival of intensive care unit (ICU) patients based on data available at the beginning of intensive care. We presented 12 fictional but real-life-based patient cases and asked the respondent to estimate the probability of 1-year survival in each case by choosing one of the alternatives 5%, 10%-90% in 10% intervals and 95%. We compared the physicians' estimates to registry data-based realistic prognoses of comparable patients treated in the ICU. Based on the difference between the estimate and the realistic prognosis, we categorized the estimates into three groups: (1) difference less than 10 percentage points, (2) difference between 10 and 20 percentage points, and (3) difference over 20 percentage points. We received 210 responses (totally 2520 estimates). Of the respondents, 43 (20.5%) were specialists working mainly in the ICU, 81 (38.6%) were specialists working occasionally in the ICU, 47 (22.4%) were specialists not working in the ICU, and 39 (18.6%) were doctors in training. The difference between the estimate and the realistic prognosis was less than 10 percentage points for 1083 (43.0%) estimates, between 10 and 20 percentage points for 645 (25.6%) estimates, and over 20 percentage points for 792 (31.4%) estimates, out of which 612 (24.3% of all estimates) underestimated and 180 (7.1%) overestimated the likelihood of survival. The median error (the median of the differences between the estimate and the realistic prognosis) for all estimates was -8.8 [interquartile range (IQR), -20.0 to -0.2], which means that the most typical response underestimated the likelihood of survival by 9 percentage points. Based on the 12 estimates, we calculated the median error for each respondent. The median (IQR) of these median errors was -8.6 (-12.6 to -5.0) for specialists working mainly in the ICU, -8.1 (-13.0 to -5.2) for specialists working occasionally in the ICU, -9.7 (-17.7 to -6.3) for specialists not working in the ICU, and -9.1 (-14.5 to -5.1) for doctors in training (p = .29). Finnish anesthesiologists commonly misestimate the long-term prognoses of ICU patients, more often underestimating than overestimating the likelihood of 1-year survival. More education about critically ill patients' prognoses and better prediction tools are needed.