Background: Low cardiorespiratory fitness (CRF) is a strong, independent predictor for chronic disease risk as well as lower work capacity. However, trend analyses of CRF in relation to different occupational groups are missing. Purpose: To study trends in CRF during the last 20 years and forecast possible future trends in different occupational groups of the Swedish working population. Methods: Data from 516,122 health profile assessments performed in occupational health screening between 2001 to 2020 was included. CRF was assessed as maximal oxygen consumption and estimated from a submaximal cycling test. Analyses include CRF as a weighted average per five-year period in 12 different occupational groups, and standardized proportions with low CRF (<32 ml/kg/min) in four aggregated occupational categories (white- and blue-collar, as well as low- and high-skilled). Also, adjusted annual change in CRF in the total population as well as by sex and age-group, also a forecast of future trends in CRF until 2040, are presented. Results: The largest decrease in both absolute and relative CRF were seen for Admin and customer service (-10.1% and -9.4%), Mechanical manufacturing (-6.5% and -7.8%) and Education (-4.8% and -7.3%) occupations. The greatest annual decrease was seen in Transport occupations (-1.62 ml/kg/min, 95% CI -0.190 to -0.134). Men and younger individuals (18-34 years) had in general a more pronounced decrease in CRF. All aggregated groups had an increase in the proportion with low CRF, with the greatest increase in blue-collar and low-skilled occupations, 16% to 21% relative change. Forecast analyses predict a continued downward trend of CRF, especially in low-skilled occupations of both white- and blue-collar occupational groups. Conclusion: There was a general trend of a decreasing CRF in all occupational groups, however the trend was more pronounced in blue-collar and low-skilled occupational groups. Structural changes at the workplaces and in society are needed to stop the downward trend in CRF. Funding: This work was supported by The Swedish Research Council for Health, Working Life and Welfare https://forte.se/en/ (Grant no 2018-00384) and The Swedish Heart-Lung Foundation https://www.hjartlungfonden.se/HLF/Om-Hjart-lungfonden/About-HLF/ (Grant no 20180636).