Aims: Obesity and weight change are associated with sickness absence; however, less is known about the diagnoses for sickness absence. We examined the association between stable and changing weight by body mass index groups with sickness absence due to any, musculoskeletal and mental diagnoses among midlife female and male employees. Methods: The Finnish Helsinki Health Study phase 1 survey took place in 2000-2002 (response rate 67%) and phase 2 in 2007 (response rate 83%). Based on self-reported body mass index, we calculated the weight change between phases 1 and 2 (body mass index change ⩾5%). The data were linked with registers of the Social Insurance Institution of Finland, including information on diagnoses (ICD-10) for sickness absence >9 days. We used a negative binom ial model to examine the association with sickness absence among 3140 women and 755 men during the follow-up (2007-2013). Results are presented as rate ratios. Covariates were age, sociodemographic factors, workload, health behaviors and prior sickness absence. Results: Weight-gain (rate ratio range=1.27-2.29), overweight (rate ratio range=1.77-2.02) and obesity (rate ratio range=2.16-2.29) among women were associated with a higher rate of sickness absence due to musculoskeletal diseases, compared to weight-maintaining normal-weight women. Similarly, obesity among men was associated with sickness absence due to musculoskeletal diseases (rate ratio range=1.55-3.45). Obesity among women (rate ratio range=1.54-1.72) and weight gain among overweight men (rate ratio=3.67; confidence interval=1.72-7.87) were associated with sickness absence due to mental disorders. Conclusions: Obesity and weight gain were associated with a higher rate of sickness absence, especially due to musculoskeletal diseases among women. Preventing obesity and weight gain likely helps prevent sickness absence.