This paper analyses the effect of unmet formal care needs on informal caregiving hours in Spain using the two waves of theInformal Support Survey (1994, 2004). Testing for double sample selection from formal care receipt and the emergence ofunmet needs provides evidence that the omission of either one of these two variables would cause underestimation of thenumber of informal caregiving hours. After controlling for these two factors the number of hours of care increases with boththe degree of dependency and unmet needs. In the presence of unmet needs, the number of informal caregiving hoursincreases when some formal care is received. This result refutes the substitution model and supports complementarity or taskspecificity between both types of care. For the same combination of formal care and unmet needs, informal caregiving hoursincreased between 1994 and 2004. Finally, in the model for 2004, the selection term associated with the unmet needsequation is larger than that of the formal care equation, suggesting that using the number of formal care recipients as anindicator of the goodness of the long-term care system may be confounding, if we do not complete this information withother quality indicators.