To date, we have studied 7 patients with X/XY mosaicism, one of whom showed an X/XY/XYY pattern. Four patients presented as newly born infants because of incomplete male development, ambiguity of external genitalia or Turner syndrome. The other 3 patients presented in midchildhood or early adult life. Bilateral gonadectomy, histologic examination of the gonads for tumor or testicular tissue, and chromosome analysis from blood and gonad specimens (and usually skin) were done in these 7 patients. The Y cell line and mosaicism were always detected in the blood culture although the predominant cell line in the majority of tissues was 45,X. The Y chromosome in one of the patients failed to show the expected bright fluorescence over the long arm, and the Y chromosome of another patient previously reported had a terminal nonfluorescing portion of the long arm. Patients with masculinization showed normal height and, on laparotomy, mixed gonadal dysgenesis. Patients with Turner syndrome showed bilateral streak gonads (2) and, in one 2 1/2-year-old girl, a bilateral gonadoblastoma. All patients with Turner syndrome were less than the third percentile in height. All 7 patients were reared as female, 4 of them requiring surgery to diminish the size of the clitoris. All 7 patients appeared to be developing normally. Nonrecognition or delay of the diagnosis, which still occurs in this condition, appears to be a result of the mild physical abnormalities in some patients and a clinical diagnosis of Turner syndrome supported only by a negative X-chromatin result.