The SimilarityLookup operation enables you to specify multiple items in a request, for example:

http://ecs.amazonaws.com/onca/xml?
Service=AWSECommerceService&
AWSAccessKeyId=[AWS Access Key ID]&
Operation=SimilarityLookup&
ItemId=ASIN1,ASIN2,ASIN3 

The response contains either items that are:

You specify the intersection or union of similar ASINs by setting the value of the SimilarityType parameter in a SimilarityLookup request. The possible parameter values are:

The second value is called Random because the similar items returned are a random pick of all similar items found. The operation can only return up to ten similar items. So, it is possible, if there are more than ten similar items, that identical SimilarityLookup requests can return different lists of similar items. It is also possible for the operation to return no similar items. This response is returned as an error message, for example:

<Error>
  <Code>AWS.ECommerceService.NoSimilarities</Code>
  <Message>There are no similar items for this ASIN: B000B776KY.
  </Message>
</Error>

An empty result is even more likely when the SimilarityType value is "Intersection."