Algodystrophy is a common entity which may present in many clinical contexts. Its early diagnosis and therapy are of great prognostic interest. Apart from the typical complete forms there are some other atypical forms, such as partial knee algodystrophy, of difficult diagnosis. Its inclusion in the differential diagnosis of gonalgia occurring in patients attended under many medical specialties is therefore necessary. Two atypical cases of knee algodystrophy in its partial form are reported. One of these cases relapsed at the heterolateral knee after 18 months of the initial presentation; this second episode was also a partial form, a fact which we have not seen reported. The reported cases are here discussed and the scarce literature is reviewed, commenting on the difficulty of the early diagnosis compared with other entities which may mimic the clinical picture, radiological and scanning features of algodystrophy, such as aseptic osteonecrosis or stress fracture. The diagnostic algorithm is discussed, pointing to the usefulness of magnetic resonance (MR) in difficult cases to rule out other entities which would entail different therapeutic modalities.