This case report concerns a 7-month-old infant with severe height retardation (-5.0 SD), typical growth hormone (GH)-deficient phenotype, and undetectable GH serum levels in response to three pharmacological stimuli. Diagnosis of isolated GH deficiency type 1A was confirmed by restriction endonuclease analysis of genomic DNA which pointed out GH-N gene deletion. The introduction of bio-methionyl-GH therapy in this patient was followed by a transient and clinically irrelevant appearance of low binding capacity GH antibodies as well as by a long-lasting catch-up growth (42.2 cm) which is continuing 44 months after beginning of treatment. This atypical pattern confirms that immune and growth response to exogenous GH in isolated GH deficiency 1A may be very heterogeneous.