BACKGROUND: Focal atrichia is a common clinical finding in female pattern hair loss, the specificity and histologic findings of which need further clarification. OBJECTIVE: To determine the frequency of focal atrichia in various types of hair loss and its histologic characteristics in female pattern hair loss. METHODS: Part 1 of the study was a review of 250 consecutive female patients seen with hair loss for the presence of focal atrichia, and part 2 examined paired biopsy specimens from haired areas versus those from areas with focal atrichia in 18 subjects with female pattern hair loss. RESULTS: Focal atrichia was seen in 46 of 104 of women with female pattern hair loss (44%), including 67% of those with the late-onset subtype versus 15% of those with the early-onset subtype, compared with in 3 of 146 of those with other hair disorders (2%). Biopsy findings of focal atrichia in female pattern hair loss showed primarily a more progressive miniaturization process than that of haired areas of the scalp. LIMITATIONS: Some women with female pattern hair loss may have had concomitant chronic telogen effluvium. CONCLUSIONS: When present, focal atrichia is a clinical clue to the diagnosis of female pattern hair loss, particularly the late-onset subtype.