Objective: To describe the frequency and distribution of new diagnoses (codiagnoses) arising during a sickness absence (SA) episode, and to analyze their effect on duration of non–work-related SA in Spain. Methods: Prospective cohort study from 2004 to 2007. Overall, 15,246 episodes occurred in a population base of 632,000 workers. Median duration of the episodes was the measured outcome. A Cox survival analysis, stratified by sex, estimated the hazard ratio to case closure adjusting by initial diagnosis, codiagnoses, demographic, and employment-related variables. Results: The effect of an increasing number of codiagnoses on duration of the SA episode was evidenced by a progressively decreasing hazard ratio. This was present in both sexes, although the effect was greater in men. Conclusions: New conditions arising in the course of a SA episode significantly prolong its duration.