Patients with Kallmann syndrome (KS) exhibit hypogonadotropic hypogonadism and anosmia [Kallmann et al., Am. J. Mental Def., 48 (1944) 203-236] secondary to failure of gonadotropin-releasing hormone (GnRH)-producing neurons to migrate from the olfactory placode to the brain, and to agenesis of the olfactory bulbs. It has been hypothesized that olfactory neurons (ON) from individuals with KS are immature partly on the basis of studies in animals showing that lack of synaptic connection of ON with the olfactory bulb results in expression of immature ON [Schwob et al., J. Neurosci., 12 (1979) 880-883]. To test this assumption, we obtained olfactory tissue samples from two males diagnosed with KS on the basis of medical history and MRI studies. Both patients were anosmic. The functioning of cells isolated from biopsies taken from the upper middle turbinate and septum was studied by measuring changes in intracellular Ca2+ concentration ([Cai]) using dual excitation fluorescence microscopy. Biopsies from both patients yielded cells that morphologically appeared to be ON. Seven of 16 cells that morphologically resembled ON responded with a change in [Cai] upon stimulation with an odorant mixture. These studies show that at least some ON in KS individuals are functionally mature and suggest that complete development of the olfactory bulbs is not required for differentiation of mature human ON.