Objective: To evaluate the predictive utility of the number and morphologic appearance of isolated café au lait macules (CALMs) in establishing the diagnosis of neurofibromatosis type 1 (NF1) in a cohort of children referred to an NF1 subspecialty clinic. Design: Retrospective study of patients seen between the years 2004 and 2007. Setting: Tertiary care neurofibromatosis referral clinic at St Louis Children's Hospital. Patients: The study population comprised 110 patients who presented with CALMs and no other diagnostic features of NF1. The median number of CALMs at initial presentation was 6, while the median age of the patients was 33 months. The median age at the last follow-up examination was 76.5 months. Main Outcome Measures: Number and morphologic appearance of CALMs and diagnosis of NF1. Results: Thirty-four of the children met diagnostic criteria for NF1 during the study period. Thirty-two children met criteria prior to age 72 months, and 2 children met criteria after 72 months. The mean number of CALMs at presentation in children eventually diagnosed as having NF1 (11.8 CALMs) was significantly higher than the mean number of CALMs in children not diagnosed as having NF1 (4.6 CALMs). Of the 44 children who had 6 or more typical CALMs at presentation, 34 children met criteria for NF1. Sixty-eight patients had CALMs described as "typical," while 42 patients had "atypical" CALMs. Only 2 patients with atypical CALMs met criteria for NF1. Conclusion: The majority of patients with 6 or more CALMs will eventually meet diagnostic criteria for NF1, typically by age 6 years, and this likelihood increases with increasing number and typical morphologic appearance of CALMs.