Forty-four cases of botulism occurred in infants in Southeastern Pennsylvania between 1976 and 1983. Forty-three were caused by Clostridium botulinum type B. Progressive weakness necessitated ventilatory support in 39 infants. Complications during hospitalization included otitis media in 13 patients and aspiration pneumonia in 11. Eight infants developed the syndrome of inappropriate secretion of antidiuretic hormone and two developed adult respiratory distress syndrome. One infant died of progressive bradycardia despite adequate control of ventilation. Manifestations of autonomic nervous system dysfunction recognized on admission to the hospital were constipation, distention of the urinary bladder, and decreased salivation and tearing. During hospitalization, some infants had unexpected fluctuations of skin color, blood pressure, and heart rate. Infants' strength improved despite persistent intestinal elaboration of toxin. C botulinum was isolated from seven of nine home or work environments sampled. All 44 infants were white and were receiving breast milk at the time of onset of symptoms. The majority had first feedings of nonhuman food substances within 4 weeks prior to onset of symptoms. Delineation of fecal flora in seven infants revealed predominance of enterobacteriaceae. Perturbations of intestinal flora during infancy, especially at weaning, may cause transient permissiveness to colonization by C botulinum.