In another thread Prince Lieven asked if ever in Spain a woman had been styled as Princess of Asturias in her own right. Well, Prince, I am writing here my response, all the Princesses of Asturias since Henry III of Castile (who was the first Spanish prince to be styled as such in 1388):
1. Doña María, daughter of Henry III (1402-1405)
2. Doña Catalina, daughter of Juan II (1422-1422)
3. Doña Leonor, daughter of Juan II (1424-1424)
4. Doña Juana, daughter of Enrique IV (1462-1464 and 1470)
5. Doña Isabel, daughter of Juan II (1468-1474), she became Queen Isabella I in 1474, the wife of Ferdinand of Aragon.
6. Doña Isabel, daughter of Isabella (1476-1478 and 1498)
7. Doña Juana, daughter of Isabella (1502-1504), she became Queen Joan I, the Mad, in 1504.
8. Doña María Teresa, daughter of Felipe IV (1655-1657)
9. Doña Isabel, daughter of Fernando VII (1833-1833), she became Queen Isabella II in 1833.
10. Doña Isabel, daughter of Isabella II (1851-1857 and 1875-1880)
11. Doña María de las Mercedes, daughter of Alfonso XII (1881-1904)
There were also some women that were heiress presumptive:
1. Doña Isabel Clara Eugenia, daughter of Felipe II (1568-1571)
2. Doña Ana Mauricia, daughter of Felipe III (1601-1605)
3. Doña María Teresa, daughter of Felipe IV (1646-1655; when she was formally styled as Princess of the Asturias)
4. Doña Margarita, daughter of Felipe IV (1-6 nov 1661 and 1665-1673)
4. Doña María Antonia, daughter of Doña Margarita and Emperor Leopold I (1673-1692)
5. Doña Luisa Fernanda, daughter of Fernando VII (1833-1851)
All of them could have become Queens of Spain had the King who was reigning when they were named died!