Evidence from young children's early phonological development is brought to bear on the evaluation of a newly proposed type of correspondence relation within optimality theory (McCarthy and Prince (1995), Prince and Smolensky (1993)) namely sympathy. Sympathy has been advanced to account for certain opacity effects in fully developed languages. Given the claims of the theory, comparable opacity effects are expected to occur in the course of acquisition. Toward this end, different interactions of two common phenomena-that is, final consonant omission and vowel lengthening before voiced consonants-are examined with a focus on a case study of 2 young children with phonological delays in their acquisition of English. We argue that at least some developmental opacity effects support sympathy and that such effects naturally emerge in the course of development from the harmonic ranking of sympathy over input-output faithfulness and the incremental demotion of markedness constraints.