Background Studies that assess the role of physical health conditions on separation risk are scarce and mostly lack health information on both partners. It is unclear how the association between physical illness and separation risk varies by type of illness, gender of the ill spouse and age of the couple.

Methods We used Finnish register data on 127 313 couples to examine how neurological conditions, heart and lung disease, and cancer are associated with separation risk. The data included information on medication, hospitalisations, separations and sociodemographic characteristics. Marital and non-marital cohabiting couples aged 40-70 years were followed from 1998 to 2003 for the onset of health conditions and subsequent separation, and Cox regression was used to examine the associations.

Results Compared with healthy couples, the HR of separation was elevated by 43% for couples in which both spouses had a physical health condition, by 22% for couples in which only the male spouse had fallen ill, and by 11% for couples in which only the female had fallen ill. Among older couples, the associations between physical illness and separation risk were even clearer. The association with separation risk was strongest for neurological conditions, and after incidence of these conditions among males, separation risk increased over time. Adjustment for sociodemographic characteristics had little effect.

Conclusions Our findings suggest that poor health may largely strain relationships through disability and associated burden of spousal care, and this should be taken into consideration when planning support services for couples with physical health conditions.