Scientists used an iPhone 4S to diagnose intestinal worm infections in schoolchildren in rural Tanzania.
They attached an $8 (£5) ball lens to the handset camera lens, and used a cheap torch and double-sided tape to create an improvised microscope.
Pictures were then taken of stool samples placed on lab slides, wrapped in cellophane and taped to the phone.
They were studied for the presence of eggs, the main sign of the presence of the parasites.
When the results were double-checked with a laboratory light microscope, the device had managed to pick up 70% of the samples with infections present - and 90% of the heavier infections.
The study has been published this week in the American Journal of Tropical Medicine and Hygiene.
Researcher Dr Isaac Bogoch, who specialises in internal medicine and infectious diseases at Toronto General Hospital, told the BBC he had read about smartphone microscopes being trialled in a laboratory and decided to "recreate it in a real world setting".
"Ultimately we'd like something like this to be a useful diagnostic test. We want to put it in the hands of someone who might be able to use it," he said.
"70% (accuracy) isn't really good enough, we want to be above 80% and we're not quite there yet," he added.
"The technology is out there. We want to use materials that are affordable and easy to procure."
Camera key Dr Bogoch and his team, which included experts from Massachusetts General Hospital and the Swiss Tropical and Public Health Institute, said the only reason he used an Apple iPhone was because it was his own handset.
"You need the ball lens to help with the magnification - but any mobile phone with a decent camera and a zoom function will be sufficient," he explained.
The smallest eggs visible using the smartphone were 40-60 micrometres in diameter.
"From an egg standpoint that is not tiny but it's not enormous either," said Dr Bogoch.
"The microscope was very good at diagnosing children with heavier infection intensities as there are more eggs, so they are easier to see."
Intestinal worms are estimated to affect up to two billion people around the world, mainly in poor areas.
"These parasitic infections cause malnutrition, stunted growth, and stunted mental development," added Dr Bogoch.