Objectives: Sickness absence (SA) is a widely studied integrated measure of health status. To better understand how SA behaves over time (SA trajectory) a longitudinal and individual-centred approach will allow identifying groups of individuals who share common characteristics. The aim of this study was to identify and describe SA trajectories and to assess employment conditions and diagnosis groups as determinants. Setting: Working-life and sickness absence administrative records from a representative sample of affiliated with the Spanish Social Security system. Participants: 38 420 workers affiliated with the Spanish Social Security system, born 1949 to 1969 or 1970 to 1990, resident in Catalonia who had SA between 2012 and 2014 (75 212 episodes). Results: We identified three different SA trajectories in both birth cohorts for men and women: low-stable (86.2% to 90.8% of individuals), decreasing (4.4% to 5.9% of individuals) and increasing (4.1% to 8.7% of individuals) accumulated days of SA. The main characteristic of SA trajectories was the medical diagnosis group. The increasing SA trajectory had a higher proportion of workers with SA due to mental disorders compared with the other trajectories. The association analysis showed diagnosis group strongly related with all SA trajectories, particularly SA due to mental disorders showed the strongest association with the increasing trajectory among young men (adjusted OR (aOR): 42.40, 95% CI 17.03 to 105.57). Low salary levels exhibited a strong relationship with decreased accumulation of SA days over time for old women (aOR: 2.08, 95% CI 1.36 to 3.18) and men (aOR: 2.75, 95% CI 1.77 to 4.27). Unskilled manual occupations were associated with increasing trajectories among young women (aOR: 1.36, 95% CI 1.01 to 1.84). No significant differences were observed for other employment conditions across trajectories. Conclusions: Workers with mental disorders are more likely to have increased days of SA, whereas low salary levels at later ages are related to a decrease in SA days over time. Special attention to preventing the course of mental disorders at young and middle age is warranted.